Arm action accounts for 30% of jumping power. Most of this comes from the lats. In watching NFL combiners run, I noticed in their starts and throughout the first phases of acceleration, they twisted their palms up and out away from them. In trying it, I found it gets at those lats much more efficiently.
|photo credit: bleacherreport.net|
Here are some pics breaking the movement down:
long body and arms quarter squat position, arms back
palms facing each other palms up, thumbs rotated out