When you get on a ride, you have to show your wristband, sometimes even to 2 different people. If you don't have one, they will either make you leave or let you on but require that you leave after the ride or immediatly get a wristband. We were at AK for later hours one night and we were gettin ready to get on the Triceretop Spin...the people in front of us didn't have wristbands so the lady told them they could ride but they had to get wristbands after the ride. She showed them were to get them and what do show to get them (they were Hispanic so she kept pointing to me and my family as an example cuz we were right behind them...). So I guess they ended up getting them, I'm not sure though.
To get them, you show your card (for us, our card was our resort ticket and our park hopper/premium ticket. It's really simple. Just hop in line, show the card(s), and they put a wristband on you! Then you find a EMH Guide...it tells what rides stay open late, then ride what you want to! At MK we had practically the whole park to ourselves on late hours night. It was EMH from 10 PM to 1 AM so most ppl didnt stay becuase it was too late for them or they had lil kids that wouldn't make it. So it was not crowded at all! The longest we waited for a ride that night was Space Mountain for about 10 minutes I think. It was great!