You're ... bending your left elbow (chicken-winging) through impact.
By Brady Riggs
with David DeNunzio
If you caught any of the Grand Slam tournaments in tennis this year, you saw a lot of crushing, two-hand forehands from Rafael Nadal. If any tennis player allowed his left arm to chicken wing through impact, the ball would end up 30 rows up and to his right. The trick isn't to purposely straighten your left arm, but to extend your right arm and let your right forearm cross over your left. Do that and your left arm has no choice but to straighten and then fold so you can bring the club up into your finish. In tennis, this move gives you needed topspin; in golf it gives you draw spin.