I found the description below on TP.com. I'm curious to know if anyone can confirm from actual experience.
When you approach the queue, tell the first ride attendant you see that you want to switch off. The worker will allow everyone, including young children, to enter the attraction. When you reach the loading area, one adult rides while the other stays with the kids. Attractions at some parks offer waiting areas for this purpose. Then the riding adult disembarks and takes charge of the children while the other adult rides. A third adult (or child who meets the minimum height or age requirement) in the party can ride twice, once with each switching-off adult, so that the switching-off adults don't have to ride alone. There is no cost to use the "switching off" option.
On FASTPASS attractions, the park may handle switching off somewhat differently. When you tell the worker that you want to switch off, he or she may issue you a special "rider exchange" ticket good for three people. One parent and the nonriding child (or children) will at that point be asked to leave the line. When those riding reunite with the waiting adult, the waiting adult and two other persons from the party can ride using the special ticket. This system eliminates confusion and congestion at the boarding area while sparing the nonriding adult and child the tedium and physical exertion of waiting in line.