The cerebellum is involved in coordination and fine-tuned
movement. It has many inhibitory neurons which help to make small
adjustments to motor control signals sent down the spine,
controlling movement.
In the case of athletes, these small movements can mean big
differences in direction, strength, etc. in how one makes a golf
putt, swings a bat, throws a ball, etc..
Without these fine adjustments, all of our gestures and actions
would be gross movements (flailing of arms and legs, and the
overaction seen in infants whom haven't yet developed fine motor
control yet).