Rider Switch is a simple system available on most rides at Disney World and Disneyland that are either thrill rides that some guests may not want to go on, or have significant height requirements. They’re frequently used by families with small children who aren’t tall enough to ride. The way it’s designed to work, one parent gets in line for the ride while the other waits outside with the child. Then, after the first guest is finished on the ride, they take over watching the kid while the other parent goes on the ride. Guest two gets to jump in the FastPass+ line, so as not to have to go through the entire line twice.
Source: Cinema Blend
