It's called the Baby Swap or Child Swap. Basically one parent asks the CM at the front of the ride for a child swap pass, and they'll give you a printed piece of colored cardstock. Parent #1 gets in line and rides, then gives the pass to Parent #2, who can then use the Fastpass line, turning in the pass to the CM in that line.
I've heard different variations on this, such as both parents and child wait in line together, and the non-riding parent and child wait at the front of the line while the other parent rides, then the 2nd parent is the next person to ride.
Also, you do not have the use the pass right away. If your child gets antsy and you wander away to do something else, you can come back later to use the pass.
I think you can also ask for the pass even if your child is big enough to ride, but just doesn't want to. Somebody still has to stay behind with the child, so ask for the pass.