While nothing is done to 'make' the horses buck, there are ways to encourage this ability and give the animal incentive to buck as hard as possible. This is accomplished by the use of a specially designed flank strap.
Despite what you hear from certain 'animal rights' activists, nothing is done to intentionally hurt the bucking horses.
This includes binding of testicles (a popular lie spread by certain groups against rodeo), drugging, beating, burning, etc.
The flank strap is all there is to it, and as soon as the ride is over, the pick-up men release the device.
These animals are the life blood of the stock contractors business and the rodeo cowboys livelihood. It is in the best interest of everyone that these animals are protected. While I admit that accidents do happen, there are more rules in place governing the safety of the livestock than the competitors themselves.
Most of the propaganda against rodeo is spread by the misunderstanding of these simple facts. Cowboys and cowgirls understand the power of bucking horses and a mutual respect develops between the horses and competitors.
As I have seen for myself, most horses really seem to grow to love the rodeo lifestyle. They get to remain wild and free, working only 8 seconds a day, while their counterparts have to remain under control and the saddle for most of their career.