If you've ever hung up in a doorway, on a tree, or generally found yourself falling all over yourself in close-quarters movement, you're going to get a kick out of this. "ST Movement" is a set of two tiny addons that bring a welcome change to your ArmA2 movement model.
The first, st_collision, is based on an addon concept by Rocko and Kju from the ArmA1 days, redone in ArmA2 by Rg. This addon causes your weapon collision with the environment to be significantly toned down, meaning that you can now move sideways through doorways and generally find CQB to be enjoyable, versus feel like a rusted robot trying to navigate a terrifying alien world called 'indoors'. Try it out and see, it's simple and very noticeable.
The second may or may not be a big deal, depending on how you move around in ArmA2. There is a combination of keys in A2 that, for reasons unknown, triggers you into a several-second-long evasive-sideways animation. Personally, this used to get me in trouble (or killed) frequently. The st_evasive addon simply removes this animation from the player's movement system, preventing any key combination from accidentally triggering it.
The "st_collision" may cause any side effect like clipping issues or "firing throught walls" ? Or is a win-win situation?
Just by watching the video the game seems a lot more smother. Kudos.
The A2 weapons fire from the chamber, not the muzzle, so while you can stick the muzzle into/through thin walls in theory, you can't shoot through them, due to the chamber still being on the 'correct' side of the wall. So, no, you should not see any exploits like that, to the best of my knowledge based on having played with it for awhile now.