What is "ride a buck"?

No, it's not another word for a bronc rider. "Ride a buck" is a fun class sometimes seen at playdays and schooling shows, where riders are challenged to perform ever more complicated activities (sometimes bareback) without losing a dollar (real or fake) placed between their leg and the saddle. It's seen as a test of the rider's ability to keep their seat. It's also known as "sit a buck," "bareback dollar," and various other names.