We have a 4 and 2 year old. We used it this past trip so that one parent could stay with the younger child and both my wife and I could each ride the attraction with our older child. You essentially go to the entrance to the attraction and just tell them that you want to do a baby swap. If you are going to the regular line they will give you one fastpass. The first parent goes on the ride and waits in the regular line, then when the child is swapped the other parent takes the fastpass.