It's wonderful if you have older children and one (or more) too short to ride. We love it because it allows both DH and I to ride the attraction with our older kids.
It is very easy to use:
Your whole party approaches the CM at the ride entrance and ask for a Rider Switch pass. The first part of your group rides (either through the standby line or with FPs if you have them) while one adult waits with the little one(s). (You can get a snack, ride another attraction, or just wait near the ride exit.)
When the first group is done, the parent who waited and up to two people enter the FP line with the Rider Switch pass. (It can be used any time that day, does not have to be immediately after the first group is done.)