… where’s the best place to have your support hand?
Pushing down hard on handguard/rail?
Seen it done IRL once, and it was surprisingly fast (first time I saw rapid fire in the prone position ever).
Are there other ways?
I usually have my hand on the magwell, as it’s difficult to move it much further out.
I find the best way to control the gun is to just make sure your entire body is behind it, and to have the stock in a solid position and not just barely touching your shoulder/chest.
Magazine monopod and thumb break far out on the rail…same as standing. Properly configuring your gear (if you wear a chest rig / PC) so that you can maintain the lowest possible profile when prone will make it easier to get your support hand out further.
I find the fastest follow-ups when prone come when shooting off of a pack, used as a rest with the support hand ahead of the hook of the stock (Mine are a Vltor Emod or Imod).
My method as well, its just more comfortable and easy to transition targets.
How I try to shoot, too.
If I need to elevate the rifle or something, then I bring my hand in as required. I try to keep my thumb over top the rail, but the terrain and the angle of elevation to the target required dictate what’s possible.
I get both elbows out to the sides on the ground so I have the magazine, and each elbow acting as a tripod. The mag is centerline, each elbow locks in support to each side of the rifle.
Can you go 2-3 shots per second with decent accuracy this way? Sounds precision oriented.
This would probably work with a light machine gun or similar but I think the carbine is too light and would bounce around quite a bit without the support hand applying some pressure near the handguard area.
I have been able to maintain a fairly rapid rate of fire using this method, finding that the reticle rapidly settles back onto the previously engaged target. Yep, it’s down or dropping, now shift to the next. I haven’t really timed it but fast enough for my flash suppressor to almost burn a pocket (blackened it quite nicely). The mono-pod method, using the magazine has one point of contact and a very sharp fulcrum point. Shooting off of a ruck gives the shooter a much larger point of contact on the rifle and a wider distribution of support. Shooting a 16" H-bar Recce with a magnified optic, I find it does not lift off of the ruck at all.
If you have the time and the situation permits either the formal or hasty sling and Canadian (magazine monopod) prone allows a very accurate sustained rate of fire and cadence – typified by the Infantry Trophy squad-level match (six shooters, squad leader and b-team leader):
video of a three-man fire team: http://www.youtube.com/watch?v=VbXQZpXKg4Q
six-man squad at Camp Perry: http://www.youtube.com/watch?v=Y1gAynTpNzA
If it’s speed and accuracy at any extended distance, I try to find something to rest the end of the forend on. Then I extend the offhand as far forward as possible and control over the top. Using a pack as a front rest and the mag for the rear, I was able to get 27 hits out of 28 rounds on a steel torso at 300 yards, in 15.14 seconds. Done with a Noveske 18" Rogue Hunter with a Bushnell Elite 6500 2.5-16 set at 8X. Without the support hand controlling over the top, I find it impossible to get this rate of accurate fire. The gun just bounces too much.
It is a little disconcerting to do this. It’s tough to ignore the rounds impacting the target as you’re trying to break additional shots. It messes with your rhythm! 
I dont even put my support hand on the rifle.
I shoot off a range bag, my support hand goes in my right armpit just under the the buttstock.
The bottom of the stock fits in the “V” between my thumb and finger.
Then I control movement from the back.
Define precision requirement and distance envelope.
Define “fast”: shot cadence, time to first shot, or both?
Starting position?
Area (arc degree) to be covered?
Weapon setup?
Support available?
I don’t know any tactics, so at the risk of looking like a total fool:
Torso hits inside 50 yards.
Define “fast”: shot cadence, time to first shot, or both?
shot cadence, needs to be fast assuming maybe multiple targets. I guess everyone gets served twice real quick and sweep back to clean up.
Starting position?
Hastily dropping into prone and waiting, like covering a retreat or some such.
Area (arc degree) to be covered?
Up to 90 degrees depending on buildings, cars, etc.
Weapon setup?
Basic flat top carbine with red dot. And a 2 point sling.
Support available?
None, well maybe another fool with same weapon set-up.
Note: the above is just some BS hypothetical stuff. I was just wondering how one could most quickly fire multiple shots in the prone position.
When I saw it done IRL I was shocked at how much faster the shots were compared to standing offhand, at that distance. I forgot how their support hands were positioned though, resulting in dis here thread.
By support I mean something other than your body against which the weapon can be stabilized.
Shot cadence usually referrs to multiple rapid shots on a single target, but the ability to transition from target to target is a viable and reasonable desire.
Anyway:
By your definitions I would go with a magazine monopod, hand as far forward as is comfortably possible with a clamping grip, as described above by others. It gives a good compromise between stability and lateral transition.
You can also get good work done off of a bipod with the support hand gripping a bipod leg close to the HG. It allows the shooter to load the bipod and gives good control on the front end for lateral transitions. Adding a bipod might not be the solution for most shooters though.
Both of these work best if employed with a free-floating HG, but at 50 meters POI shift will be pretty much transparent.
ETA:
The questions I asked are highly relevant to answering your question, and not at all intended to expose a lack of tactical prowess or to inflate mine.
The technique to put a bunch of bullets into a head at 500 meters with sand-bags, bipods, walls, and no other targets to worry about is best achieved by means other than what one would use at 50 in the open when time of acquiring the position is as important as the number of shots one can stuff into a torso in X seconds with threats probable from every direction.
Anything inside 100 yards should be relatively easy and quick for a skilled marksman. Remember that the more solid your shooting position, i.e., locked in prone, the slower your target transitions are going to be. For a better field of fire, your support should be closer to the receiver (inside 100, I’d use nothing more than the magazine itself for support). For greater long range precision and a narrower field of fire, the front support should be as far forward as possible.
IMO, for anything inside 100, going prone would be reserved for gaining hard cover or reducing your silhouette to multiple adversaries. I dislike giving up rapid mobility unless the advantage gained outweighs the speed of movement.
Thank you very much for these tips! I never heard of the bipod trick before, good stuff!
ETA:
The questions I asked are highly relevant to answering your question, and not at all intended to expose a lack of tactical prowess or to inflate mine.
The technique to put a bunch of bullets into a head at 500 meters with sand-bags, bipods, walls, and no other targets to worry about is best achieved by means other than what one would use at 50 in the open when time of acquiring the position is as important as the number of shots one can stuff into a torso in X seconds with threats probable from every direction.
I felt a disclaimer of sorts was appropriate so nobody will take my post too seriously. Some of the terms in your questions I haven’t really thought about before, but are definitely worth thinking about.
Squinting at the crackberry to add in a sentence or two every couple minutes (at work) is hell on my writing. Haha
At the risk of sounding like a dick, and I’m not trying to, why do you need a 2-3 shot per second cadence from prone? I gained from this thread because, while I do use the mag monopod when appropriate, I have always done so in the traditional underhand support hand grip. I have grown more accustomed to the thumb over grip as of the last couple of years but have not used that grip when prone. So it gives me something to try out. And regardless of cadence chosen, being setup for the followup hot faster is always preferable.
BUT, I am not a big fan of prone inside of 100 yards as it limits mobility (especially if ur alone) and you should be able to get good hits from a knee at that distance. Outside of 100 yards, other than very limited scenarios, a 2-3 second cadence is too fast in my opinion. I see no civilian purpose for engaging threats at this range other than some shtf apocalyptic scenario. But you would either be shootin to keep the ememy’s head down (or off :)) to advance or break contact when you are under effective fire. Either way, unles the engagement is going to be predictably short lived, 3 shots a second = 180 rounds a min = 6 30rnd mags. One damned minute! Unless rare circumstances are present, for the 100 yard and out shots, I think a cadence less than one shot a second will get you in trouble.
I know these are imagined, made up scenarios and it is good to improve range skills for the sake of marksmanship but I would say that inside 100 yards, think hard before going prone. And outside of 100 yards, slow your cadence to preserve your ammo, ammo management is more precious than cadence sometimes.
So my disclaimers aren’t that useless after all…
Yes, probably very limited in application. I was wondering only about the technique.
Thinking out loud, one could use this shooting rapidly from prone while being behind cover. Much smaller target area, and you can roll (is a simple roll too ninja?) back behind cover should things get too exciting.
Then there is the concealment/surprise factor, of course this wouldn’t be a good position to assume when the enemy/s have already seen you. 
More like a quickly set-up ambush on the advancing targets or perhaps for defending a position. Rate of fire would depend on distance/capability of course.
What if your leg/s get hit and you could only shoot effectively from prone and have to do it fast? 
IMO If we train weak hand only shooting, and one-armed reloads, etc, this (shooting rapidly from prone) doesn’t seem too far fetched.
Again. Probably not something to do in a middle of a field with no cover/concealment, after being spotted by baddies, with nothing on but a carbine and SpongeBob underoos. lol
looks fun!
“We’re weiners. We’re weiners.” 
Hahaha, now we are having fun! :). I have nothing against imagination and like I said, improving range flat range shooting skills is useful.
I don’t like to discourage folks, my point was about about what I consider practical application of prone and cadence of fire under more realistic conditions. I have no problem with people thinking outside the box, that is how new thoughts emerge.
My opinion is that one should train for the high percent solution, low percent solution, and even the very low percent solutions to some degree. Anything lower than that on my scale, such as needing to lightning ambush zombies from cover by utilizing a ninja roll with a festering zombie bite to the leg reducing mobility … well, that kind of stuff goes in the adapt and improvise category.
I’m not ripping on your ideas and training to improve your shooting skills is a noble goal. Like I said, the responses to your topic have given me a new prone technique to try out.