I think it has to simulate the effect of someone being hit by a bullet, but how does it work?

Thanks!

If it is what I think, it is just a harness that an actor would wear. At the moment of "impact" the rig is jerked, giving a ragdoll effect to the actor. It simulates a more realistic impact than the actor can deliver. Anytime you watch a movie, and someone gets kicked through a wall, etc., they are likely wearing the rig.