You make a "reservation" for the attraction of your choice via the Flash Pass device so it will hold your place in line for you. During your wait time you can do other things (Eat, shop etc...)
When it's your time to ride an alert will be sent to your device. Each flash pass enabled attraction has a dedicated entrance for FP users. (Usually via the attraction's exit)
Once you reach the loading platform an operator will "scan" your FP and you will join the line on the loading platform.
The Platinum FP offers the least amount of waiting and allows you to ride twice without getting out of your seat.
Hope this helps. Flash Pass Info