They are held in by a balloon or have a little wire in them then when pulled on, curls the part in your stomach up like a little pig tale keeping it from coming out. The balloon type is more obvious in its function.

I was not near my docs when it was time to 86 mine, so I cut the little wire, and it just slipped right out when the pig tale became straight. No pain, no nothing. A 2X2 taped over it for 3 days and it was closed and looked like another little belly button. I don't recommend that you do this; I only offer the story here to show you how simple the process is, and that it is nothing to be afraid of.