Mid Northern Rodeo club president Noel Upton explains.
"It's like a competition between the contractor and the cowboys.
"If they get 100 per cent buck-offs, the cowboys get no money and it all goes to the contractor.
"The idea is to get bulls that buck the cowboys off, and they get a no-ride and the contractor does well."
Each bull has a general pattern of bucking that it uses at rodeos, and cowboys try to improve their chances of success by learning the pattern of the bull they are going to ride.
"It gets to the stage where cowboys want to draw a good bull and they want to know how to ride it," Mr Upton said.
Bull riding is one of the most exciting and dangerous of rodeo sports. The rider grips on to a rope tied around the bull and tries to stay on for eight seconds, while the bull does everything in his power to buck the rider off.
Past club president Murray "Gramps" Darroch reckons if you don't get a buzz watching a cowboy ride a bull, there's something wrong with you.
"It's a great adrenaline rush, when you get really good bucking stock and a really good cowboy," he said.
"It gives you an adrenaline rush just watching them.
"Some of these bulls weigh about a tonne, and you've got a man weighing about 80kg, and he's putting his balance and strength against a tonne of meat."
Northlander Klay Lanigan is one of those who will be jumping on the bull's back at the Mid Northern Rodeo this weekend.
The 15-year-old from Okaihau is one of a six-member New Zealand under-18 team who will be competing against their Australian rivals in the Transtasman Challenge.