Think of the parent switch pass like a fastpass that never runs out and that you can use anytime. First you approach the CM at the beginning of the line, and display your obviously under-height preschooler, they hand you a little slip that looks like a fastpass, but says something like "rider switch" on it. One of you then gets in line (regular or fastpass return, if you have one), and the other takes your son to do whatever (snack, ride something else, etc.). When rider #1 finishes, the parents switch, and rider #2 can ride, usually by entering throught the fastpass return line.
Tips and advice: You don't have to use the rider switch pass immediately after rider #1 rides. If your son is bored/hungry/tired or you have reservations or the parade is about to start, you can go do something else and rider #2 can return any time that day to ride without waiting in line.
Because rider switch works just like fastpass, don't bother getting more than one fastpass for any attraction where you'll be using rider switch. For example, you can get one fastpass for Splash Mountain and one fastpass for Big Thunder Mountain Railroad. When you show them your son and get the rider switch pass, it's just like getting a second, free fastpass.
Along those lines, you can also use your son's ticket to get a third fastpass for another attraction, then turn it into two by getting the rider switch. They don't care whose ticket was used to get the fastpass, or who uses the fastpass. The only time you'll need more than one fastpass for the same attraction is where you'll all 3 be riding together.
You also don't necessarily have to switch riders. If rider #1 is a thrill seeker, but rider #2 has no desire to ride California Screamin', rider #1 can ride first, and then use the rider switch pass to ride again.
Finally, though it doesn't sound like you have older children, rider switch is awesome for older siblings because, as someone else mentioned, the rider switch pass is good for 3 or 4 people. So dad rides with big sister while little brother sits out with mom, then big sister gets to ride again with mom while dad sits out with little brother.
Well, that went on a little long, but hope it helps!