“Your waist rises as you twist and falls as you overturn. Rise equals go, fall equals strike. Together they mean to strike like a rolling wave. Each part must be clearly differentiated; all must be done like lightning. This is facilitated by keeping the body relaxed until the final instant.”Actually, this is from a book on the Chinese internal martial art Hsing-I by Robert W. Smith (Hsing-I: Chinese Mind-Body Boxing, 2003).
Had I not revealed its source, practically all ITF practitioners would have agreed that it is a reasonable description of the “sine wave motion”. I have written in several posts in the past that the principles taught by the so-called “sine wave motion” are hardly unique to ITF Taekwon-Do, as this quotation clearly demonstrates. (You can read a previous mention of the similarities between ITF and Hsing-I here.)
A main reason ITF Taekwon-Do's “sine wave motion” is so controversial is because the term is a misnomer -- it is not an actual sine wave. It may have been an attempt by General Choi who proposed the term to make it sound more scientific. We should keep in mind, of course, that English wasn't his first language, not even his second language (that was Japanese), and neither was he a physicist. I have heard accounts that he adopted the term after that is how someone else (an English speaker) described what General Choi was trying to explain as a “sine wave motion”. I don't know if there is truth to this, but I do believe that if another term than “sine wave motion” was used, even if it was just called “wave motion”, it would have been less controversial.
To get back to the quotation above, it very accurately describe how the so-called “sine wave motion” is generally used for something like a middle punch: The body should be “relaxed until the final instant,” the rising portion is when the waist is pulled back, and it is during the falling that the strike happens.
In another post from long ago, I looked at another Chinese internal style, Chen style Tai Chi Quan, and noted some similarities with the way we understand movement in ITF Taekwon-Do.