Leverage and timing are as important as strength. What makes a winner is the teamwork between horse and rider to get a straight running steer. A horse that changes direction. Every change scores points. The rider who touches the horse with his free hand is disqualified.
Judges primarily watch for spurring action, with every jump or buck the rider gains points.
The higher the kick the better the score. The rider is disqualified if he isn't in position for the first buck out of the chute. Spurring at the shoulder with every jump ads points. The partnership between horse and rider is important. Time is important but if you break the barrier, the start line, a ten second penalty is added to the time.