According to the researchers, freaking out is primarily associated with the left hemisphere of the brain, while the right hemisphere deals more with mechanical actions. Meanwhile the cortex of the right hemisphere controls movements of the left side of the body, and the left hemisphere controls the right side of the body. So they figured that if you can purposely activate the right hemisphere — in this case, by making a fist or squeezing a ball with your left hand — it will improve physical performance and draw focus away from the ruminating left hemisphere.
If you’re old enough to remember: Joe Morgan of the Cincinnati Reds used to flap his arm when he was at-bat, a trick he said was a reminder, from one of his coaches, to keep his elbow up. I wonder if that ritual also could have had this distraction effect.