The moment an arm technique is executed—whether defensive or offensive—the other arm moves back in the opposite direction. In karate, this is called hiki te: the withdrawing hand. The hand being pulled back moves with the same force and speed as the active hand, thus reinforcing the action through the countermovement.
Effective training methods for this include exercises that focus on the back hand to get used to actively controlling the hiki te. It's even better to practice the hand retraction separately, without performing any forward movement. In this case, the arm that normally performs the technique simply remains inactive. It's the principle of cause and effect: every action has a reaction.
