There was a G+ question about how power is generated various styles of martial arts on 2013 May 11. Here was my response, from the kung fu paradigm:
Power generation comes from what’s called “spiral force” in kung fu. By rotating the waist (in addition to maintaining other harmonies), force is transferred from the abdomen to the fist (or foot, shoulder, etc.) This is most often expressed in the phrase, “Striking force begins in the heel, is guided by the waist, and manifests in the fingers.”
From what little I know, Xingyiquan has more emphasis on shoulder rotation than waist rotation. The Three Body Stance (the most common internal force building exercise across Xingyiquan schools) is very good for developing and consolidating internal force at the arms and shoulders. This makes a good amount of sense; practice exercises that develop force that you use often in your art. Baguazhang force training, for example, has much in the way of waist rotation and developing powerful palms (characteristics that are emphasized in Baguazhang combat application).
The foot stamping (which can be seen in many Xingyiquan practitioners who bring their back foot up to the front foot when delivering a crushing fist) also helps shoot force from the whole body to the fist. This method sees frequent use in Chen family Taijiquan as well.
Ah, regarding the subtleties of the weight and pressure changes, I’m afraid I’m the wrong fellow to answer that. My training in Shaolin kung fu and Baguazhang emphasizes smooth footwork with an overall “evenness” throughout.
I know that Xingyiquan’s stamping and pressing footwork is a bit more explosive (similar to some of the patterns in Chen style Taijiquan, especially in the pattern Immortal Pounds Mortar) and that the stamping (especially when moving from the Three Body stance to the T-step) helps project force from the entire body to the fist, but I do not know all of the details. I just know that I am able to feel it happen and that my brother’s sifu emphasizes that movement in Crushing Fist (beng quan), but doesn’t explain the detailed mechanics (like most traditional masters, he’s more concerned with achieving the skill, in this case exploding force, rather than the deep how-to’s and physiological why’s).
Now I really miss my aiki jujutsu sensei. He’s a Ph.D. in anatomy and physiology and would be exactly the man to tackle this question.