How various airlines implement this varies. They CANNOT require you to do your reserve time on your scheduled days off, although a few rogue chief pilots have tried this in the past.
Rickair - thanks for the info... I've also wondered about this as I try to decide what to do when I grown up. I notice you used the word 'require'.... do the airlnes expect you to schedule some of your reserve time during time off? In other words, would it be poo-pooed to schedule, say 6 days of reserve duty after a 4-day break (or something to that effect). I just wonder if flying for airlines and the guard is more like having two part-time jobs or two full-time jobs.