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Ki is rattling a bag of chicken bones in the eyes of some. I have a view of Ki, as a physical cyclic phenomenon that has a demonstrable physical basis -- and it really does rattle a bag of bones, but no chicken is involved -- unless you're just into that kind of thing -- not that there's anything wrong with that.
In response to a question about impulse of a strike it struck me (pun intended) that it is a matter not just of length of time, but of timing (in the sense of rhythm) in the frequency of the strike at impact. It matters -- not merely speed, but the actual frequency of the impact/react timing.
It requires some teensy background on force, momentum and Newton's Third Law (action= reaction).
The commonly understood linear equation F = ma can be rewritten as Σ F= dp/dt where the sum of forces is the change of momentum (p) with respect to time (t). Impulse, J = F * dt = dt* F = dt*dp/dt = dp, which is simply the change in momentum.
Very, very rarely can real world forces be treated as purely linear. Most forces are usually:
1) not isolated (there are other forces in play, with different 3D vectors),
2) dynamic, and the amount of force is changing rapidly with respect to time, and
3) eccentric (off center) as to both or all objects involved, thus involving moments (potential rotations) or actual rotations (angular momentum).
When an object is struck what strikes back (3rd law) is its moment of inertia -- its inherent reaction to having its center moved or its mass moved about about its own center. While this is often treated as linear inertia of the center of mass, that is in fact a very special case, since any object that has appreciable volume, is always hit at some distance from its actual center, creating a moment.
The object has its own inertial moment(s) that is being moved about its own center. If you hit the one end of an object that is in free space (momentarily detached from its support) it rotates about its own center in reaction -- i.e., it does not "hit back" in any linear sense, rather the inverse motion of its OWN inertia about its center creates the rotation reaction of the opposite side of the object in the opposite direction from the eccentric applied momentum (satisfying the 3rd law), rather than a linear "resistance" reaction and the point of contact. Thus, in angular momentum terms the "reaction force" is resolved internal to the object through a rotation. Or in a complex linked body (like ours) it is resolved through many, many possible cascades of alternating internal rotations (waves) in three dimensions (spiral waves).
Any wave has a frequency, and all objects have a resonant frequency at which they will harmonically and sympathetically vibrate. That represents the elimination of any internal damping of momentum in the wave being applied so that a resonant cyclic momentum reaches all part of the structure without appreciably diminishing - whereas a purely linear momentum does not.
Inversely, if you provide a certain shape and rhythm of impulse (defined as a change in angular momentum) to one part of an object (at the resonant frequency -- equivalent to a rate of application, or the correct rhythm or maai, pick your terminology) that angular momentum propagates through in a harmonic fashion with minimal internal damping, the spiral rotation reaches the extremity supporting it -- and in the anti-phase of the wave -- detaches the object momentarily from its support (overcomes briefly the moment of gravity and the entire body of the object is then instantaneously free to rotate in space in response to the positive phase of the pulse. And if the wave is spiral, then the positive phase is offset from the anti phase by 180 degrees( of the spiral's rotation, but not necessarily in the same plane as the anti-phase at the time it affected the detachmentnof the support -- i.e. the turn of the spiral can carry the support outside of its limit of lateral stability
In short you can pound people so as to drive them in the ground, in which case you are merely hitting earth, or you can pound them so as to unstick them from the ground -- if you do it right -- and it has less to do with gross angles of strike than with the frequency of impulse at impact. Bone rattling strikes are exactly that and they can be relatively slow or much more energetic as long as they are some harmonic of the fundamental frequency of the body.
Thus, it also has less to do with the magnitude of the movement, or its absolute energy. Since the human body is in a constant struggle with gravity its own movements play in to the resonance cascade an things like Ikeda Shihan's infamous wrist twitch drops on big burly guys become much more comprehensible. It is harmony like in music -- because it IS music. And it must be learned, like music, intuitively and through much practice.
The resonant frequency of the human body is found in studies to be about 10 Hz. You can easily determine this for yourself: Perform tekubi furi at the rate where your middle vibrates up and down and count the cycles in one second. Do the same for furitama at the rate where you feel your center vibrate in time with the motion of the hands. It's right at 10 Hz for both actions, and it alternately lifts and drops the weight off your heels.
With real people there are biomechanical things (that relate in a deep way to this point) coming into play like exploiting ones own and one's opponent's autonomic stretch reflexes to different advantage -- but that is another topic of discussion .