Sometimes, a different perspective on a very well known issue reveals completely new insights. Jeff Christian from the United States sent us a good example of this principle. His excellent Makiwara Monday video was shot from below. This angle gives you a very good perspective to understand that the body and not the arm does the work during a Shotokan tsuki.
As you can see: Jeff is not using the muscle power of his arm to hit the wooden board. Instead, he is using his upper body to initiate and accelerate his arm. Thus, his arm becomes like a whip. This whiplash-effect creates force without consuming much energy. The board behind the Makiwara reveals how much strengths Jeff can generate with his tsuki technique. Very good job, Jeff!