What is a bucking dummy?

Was looking through videos today and found this one:

This is a stock contractor testing young horses as bucking stock and that weird thing on their back is a modern bucking dummy.

These are also used to test and train bulls. The dummy is designed to be bucked off, just like a rider, and in this case the contractor is deciding which of these young horses has a good enough bucking reflex to make a rodeo bronc - and which will be sold to be trained as riding horses. The dummy can also be released remotely, so the trainer can control when the "buck off" happens.