Yes, they are made so that there is a pocket under their vent to catch the poo which you put some sort of absorbent something in: a bit of sanitary pad, one of those makeup remover pads, whatever, depending on the size of the bird and the pocket.
I can't really say I've used one. I have one and have put it on a chicken but never really left it on very long. It freaks them at first and they squat and try to run backwards out of it. Since I was dealing with a 5 week old, I didn't push it. And she didn't poo in it either. Same experience with my cockatiel. I never pushed it with either of them.
I'd be interested to learn of anyone's experience with them too that has really used one.