Roger,
I asked the chief instructor and a senior instructor of our system about the question you posed, and here are there replies:
I don't know that I can help insofar as providing an explanation with references to specific laws of physics, but...
In the case of the bag flying away from you (or your opponent) when you strike it/him, this demonstrates that the force of the blow is dissipated along the outer surface of the object to create a "pushing" effect.
"Shock" is a great deal of force concentrated onto a very small point which deeply penetrates the target instead of being spread over a large surface area. There is no "pushing" effect, so the target/victim doesn't fly away from you. A heavy bag will either bounce up and down or "scissors." A human will usually just collapse due to internal damage.
If you hold the muzzle of a high-caliber weapon against (or very close to) a bag and fire it, the extreme shock will cause the bag to jump or "scissors."
I think it has everything to do with a large amount of force concentrated onto a very small surface and delivered with sufficient velocity. This allows for deep and very quick penetration.
If the force is spread over a larger surface and/or lacks adequate velocity, more of a "pushing" effect is the result.
A human body can only move so many ways to maximize both it's mass and velocity and we call these the "body actions." There are no other ways to do this, regardless of what style or art someone practices.
Does that help?
Sifu Starr
The other reply:
Well...the difference is in the ballistsics of the punch.
So, if you are looking for a physical explanation, that is the area you need to look at.
Velocity and focus are key to executing a finishing blow...
Yes, you can punch someone and move them a good distance but what this demonstrates is a force that is decelerating as it meets the target.
The target absorbs a fair amount of the force but there is no shock in the technique.
The technique you describe (where the bag pops in place or the opponent drops in place) is maximizing velocity and mass with a focus that allows transfer of force over the shortest time possible...hence, big shock, little movement...It is like the difference between a muzzle loader 50 caliber round with a single powder charge versus a 50 caliber fast burn powder with a plechette round...One packs a hell of a punch IF it can penetrate the surface (muzzle loaded round)...the other will pierce armor and create a shock wave that causes an "implosion" type injury...
When we demonstrate the force of an internal punch, we use the less lethal version and move our training partner back...
When we demonstrate the force of the "shock" we are capable of...we reduce the velocity and the mass so we don't hurt the training partner.
BUT, the focused strike IS quite obvious to the receiving party.
So, when you want to discuss the "physics" of a punch...look to the literature on ballistics...it is the closest thing to really describing what we do.
chufeng
I hope that these explanations help out. Have a good one.
Vince