When performing this exercise you should be using the Wu Huo - Martial Fire Chi Kung reverse abdominal breathing technique.

Inhale deeply as you extend your arm out in front of you and then from this position exhale sharply while pushing out the abdomen with as much explosive power as possible at the same time as making ther clawing motion as if you were grabbing hold of someones arm. Try to feel the chi moving up from your dan tien to your hand and adding to your strength. Keeping your forearm and fingers tensed as tightly as possible you should bring your hand back to be chambered at your hip whilst gradually closing your fingers to form a fist. Only relax your fingers once your fist is chambered and you are ready to inhale and begin the exercise again. You should start with a relatively small number of reps (say 5 or 10 on each side) and gradually build up to a larger number.