If it's all body mechanics then why bother telling her to "drive it through his head"? Why say anything at all? If it's all the work of muscle groups working together why does he need to direct her to perform any mental gymnastics?
Also, the idea behind unbendable is to perform it with a relaxed arm and focused intent (coordination of and and body, a.k.a. correct feeling or as Dan posted in #50 "controlled use of the body systems through intent"). When performed correctly unbendable arm can be held for an indefinite period of time. Reliance on muscles alone will eventually lead to the arm bending as the muscles holding the position relax on their own when they become exhausted.
Yes it is body mechanics but the body can't do anything without the mind telling it what to do, the mind leading the body. The mental gymnastics are to let the unconscious part of your mind coordinate the body systems, lead the body. A lot of limitations are put on the body when the conscious mind coordinates body activity.