say if you walk up to an attraction and it says a longer wait than you want.
You then decide if you want to wait or express pass it.
If you EP it, you then take your ticket and scan it through the machine.
It will give you a choices of three (I think?) times that you can return. Pick the one most conveinent to you (there is a touch screen on the machine,) and after you pick your time you want to come back, you will have a paper slip stating the time you picked.
When you time is ready, just walk in the Express line with your paper slip with the time on it. (Its sort of like a reservation for the ride!)