The medial shorter heads are activated by downward push. (fitnessandpower.com)
Newton's third law («equal and opposite reaction») says the air getting pushed downward pushes the plane upward. (discovermagazine.com)
The two most common mistakes new riders make are always «mashing» or focusing only on the downward push of the stroke, and «toeing» or pushing only with the front of your foot and not driving down with the heel. (sportstarsmag.com)