It sits between the barrel and the breech, and the fins bend inward as the ball feeds into the barrel.
It won't fall out unless you remove the barrel and tilt the breech end towards the floor, as the bent fins will keep it in place.
I used a prototype this season, and didn't have any issues. It kept balls as small as .676 from rolling out of my bull barrel.
It IS a pain though, if you have to swab the barrel for any reason. I have to the remove the barrel, then the wedgit, before running a swab through.
The only other option is to remove the bolt and insert the swab from the rear only, so that the fins don't get bent. This doesn't work if the barrel and body length is greater than the swab itself.
The back block also adds to the level of difficulty. It's all good though if you never get a chop or barrel break.