Like Hamsterstyle said, essentially your belt keeps it in place. It does shift around a bit, which I consider a plus - it makes it a bit more comfy when moving around, sitting down and getting up, so forth.
There are two options for pulling the trigger cover off: one is a lanyard that you loop around a belt loop, the other is the plastic loop with the one-way snap, which is what I use, and probably limits the shifting around of the pistol a bit more. I find it overall to be the most concealeable of the AIWB holsters I have tried because it has the least material to it. It just rides flatter against my body, takes up less room.