The FASTPASS system allows guests to avoid large lines by scheduling a specific time to go on a ride. Guests insert their admission ticket, annual pass, or pirated FASTPASS activator (downloaded from the Internet) into a FASTPASS machine near a FASTPASS-enabled attraction and receive a FASTPASS ticket with a time stamped on it. For example, a guest might get an Indiana Jones FASTPASS with a "1:30 to 2:30" time on it, or a Space Mountain FASTPASS that says "Come back in 2005." After receiving the ticket, the guest is free to go spend money elsewhere in the park until their time slot opens.
At the appointed time, the guest returns to the attraction, surrenders the FASTPASS, and is either allowed on the ride immediately, put in a short line, or laughed at for getting a FASTPASS for a ride that there's been no line for all day. If a guest misses the appointed time slot, they're are out of luck and will be sternly warned and/or glared at for attempting to use an expired ticket.
The FASTPASS system is quite complicated, and is designed to minimize the time guests spend waiting in line while encouraging guests to experience less popular attractions (e.g. Disney California Adventure). There are a number of rules the FASTPASS computer uses to calculate a rider's time slot. For example:
The time slot is always one hour in duration, except for attractions with exceptionally long lines in which case it is half an hour in duration, unless there has been unusually low attendance in which case it is 90 minutes in duration
The beginning of the time slot is calculated by taking the average ride throughput since park opening, comparing it to the historical average throughput, plotting the current wait time at attractions within the same area of the park, and then adding an hour to the current time
If the guest requesting the FASTPASS already has an unused FASTPASS then they can not get a new FASTPASS unless:
They are within the time slot printed on their existing FASTPASS,
The attraction they are requesting a FASTPASS for can be experienced without interfering with the time scheduled on the existing FASTPASS, or
The morning shift forgot to connect the FASTPASS machine being used to the FASTPASS network.
If the wait for the attraction is unusually long, a "bonus" FASTPASS is dispensed, giving FASTPASS access to an under-used attraction at the other end of the park (e.g. the Mission Tortilla Factory or Treasure Planet character meet-and-greet)
If the total number of FASTPASSes given out on a given day exceed the attraction's daily ride capacity by more than 50%, then the FASTPASS dispensed will indicate a time slot on the next day on which a time slot is available, and the guest will be notified that a separate admission will be required when they return to experience the attraction.