Bruce,
I looked around online but could not find any useful information.
The belt is captured by the pins, guides, or formed metal parts. When the belt is driving, the belt only touches the pulleys. When the belt is declutched, or slackened, the slack is forced to loop over the driving pulley and not touch it.
If I have your email address I could take a picture off my Simplicity tractor and send from the cell phone. Most mowers, garden tractors, etc that use a belt to clutch will have this arrangement.
Bob