There is a lot of confusion regarding body shaking during energy practices, the A&P, and the many other situations in which shaking can spontaneously arise.
I will explain what the shaking is, in my view.
If you pay attention, you will notice that shaking coincides with memories arising and passing. These are memories you have now found equanimity with via your practice. They were usually troublesome in nature, and were built into the system as a "block" due to resistance to that experience at the time.
Memories / emotional imprints are stored as muscle tension patterns across the whole body. The solar plexus area appears to be the epicentre, or "distribution network", of these muscle tension patterns, hence why shaking appears to emanate from here most often.
In actuality, muscle tension patterns can occur anywhere in the body. For example, releasing a blocked leg or arm muscle via posture work will spotaneously release the memory associated with when that block was formed. Whenever I do posture work I always, without exception, experience vague, often distant and seemingly unrelated to anything, memories, drifting through my awareness and vanishing as the muscle is released. If an arm is released, the shaking will nevertheless move into the solar plexus area as the memory release occurs. Again, this area appears to be the "distribution network".
However, the solar plexus area itself, and indeed all the muscles associated with breathing including particularly the diaphragm (hence breath of fire work) are major, MAJOR stores of muscle tension patterns.
When, via your insight work, you find equaniminity with certain experiences (or general "themes" of experience formed by the summation of many individual experiences), the muscles will begin to unlock as the full emotional weight of that experience is now allowed by the unconscious to be experienced by the mindbody. You will perceive this as energy flow in that area.
The shaking then kicks in as this is how muscles dissipate stored tension.
I recommend that, rather than allowing violent shakes (I can enter full seizures at will during this type of work), that instead you stymie the unblocking process, slow down the shaking and let it flow at a lighter pace, and simply allow a much longer period of time of gentle shaking to allow a full dispersion.
It is also important that you investigate the sensations associated with the release, using insight meditation on the areas of energy flow, while this is going on. The areas of energy flow will tend to move. Follow the movement with your awareness. This additional practice is equally as important as the shaking itself, as this allows the insight released from the "locked" experiences to be registered by the conscious mind.
To allow the most unimpeded flow, relax your face and simply choose to "be" with the sensations of the release in the body. Facial contortion, wincing etc., is actually a key part of emotional repression, and therefore a key part of the resistance process which initially created the blocks (this is why you wince when you see something unpleasant -- the face can actually inhibit the rest of the body from experiencing the full weight of the emotional experience).
I hope this clears up all your shaking questions and can really help you refine your practice. If you have any questions however, please ask!
