How it works:
Horizontal stability is achieved by the three ‘fingers’ that go into the rear sight mount (as you can see in the video it’s very tight). The factory rear sight is removed but the spring is retained.
Vertical stability is achieved by the top cover being pressed up into the carrier return spring by the rear sight spring, or at least that’s how I understand it… 
You can move the top cover slightly up and down but it always returns to the same up position due to the spring pressure.
Testing:
I zero’d the TRS 25 on my SGL-31 then ran 90 rounds through it, removing the top cover after each magazine then I fired the test shots and groups (shown in the video) at 50m while removing the top cover and banging on it from a few different angles.
I then moved back to 100m and removed the top cover again and took a few shots at a 10’’ steel plate ––-all hits (again shown in the video). After I turned the camera off I continued to fire/remove the cover several more times throughout the day.
Pros:
-Maintained zero throughout testing.
-Fits most AK variants (not Yugo/Serbian variants)
-Easy installation. Hex wrenches were provided and the directions are pretty straight forward.
-The finish on the product is even and generally looks good. It matches the Arsenal paint pretty closely.
-Made in the USA.
-It has a lower 1/3 co-witness with micro dot type red dots.
-The rear sight is integral to the unit so you don’t have to purchase a separate one.
-It allows for easy cleaning after shooting corrosive ammo as opposed to gas tube type optics mounts.
-Reasonably priced.
Cons:
-The top cover rubbed against both the Krebs safety used in the review and the factory safety. I have since took a file to the inside of the top cover eliminating the issue but I’d like to see it modified like the Romanian top covers to eliminate this.
-I’d like to see the rear sight moved to the rear of the unit allowing for a longer sight radius.
-When removing the gas tube the lever gets stuck on the ‘fingers’ of the front of the rail (hard to explain in writing but I demonstrate it in the video and you can see the marks in the photo above).
In the end, none of the cons would stop me from buying another but everything can be improved. The most important thing most of you are likely concerned with is whether it maintains zero and mine certainly does.
Here’s the video review so you can see it in action and explained much better than my literary skills allow:
