Kriya is deeply stored cellular information of action on the karmic level within the physical, emotional, mental and energy bodies.
They manifest physically on the external level as involuntary jerks, spasms, shaking, trembling, swaying and jumping. They can occur on the mental and emotional level, taking the form of uncontrollable crying, laughter and screaming, speaking in unknown languages. Internally they can make buried emotions and thoughts to surface from the regressed subconscious level, past forgotten memories accompanied by electric vibrations, hot or cold sensations and pains along the spine, visions and out of body sensations, internal sounds and smells.
In yogic terminology kriyas are element purifying movements pushing through blocks in the flow of kundalini. They are a byproduct of Pranic movement in the subtle body and a salient feature of the Shaktipat transmission process.
Yoga and the ancient scriptures was reverse engineered through observing and recording various spontaneously arising self regulating breath rhythms (Pranayam), spinal locks (Bandhas), stretches and body arranging itself into postures (Asana) and gestures (Mudras). The vast database of yogic movements are not random, these involuntary arising positions created by kriyas were observed for thousands of years.
Whether a voluntary complying with the way the energy wants to go, or involuntary compulsory asanas forcing the flow in conscious recess , both serve the purpose of unfolding and animating new patterns of subtle energy and its pathways.
Unfolding evolutionary biology
The external developments can be due to the sympathetic nervous system, and the internal implosions due to the para sympathetic aspect of the autonomous nervous system.
Often they can be seizure type impulses related to certain neurotransmitters. They maybe even be misdiagnosed as epileptic seizures by someone who has no knowledge of kundalini. The difference between the actual medical condition and spiritual expression is that in the later, one is fully aware and conscious while in the medical condition it is deficiency in the brain which causes complete loss of consciousness and all senses.
The spontaneous convulsions and contorted movements produced by kundalini are nervous stress being released in order to unblock and restructure the tissue to convey greater consciousness. Although they can also be seen as convulsive discharges which cause spontaneous firing in the neuro motor areas of the brain , enhancing the brain chemistry and neural mapwork to evolve for a higher vibration, bioelectric capacity.
Refinement & recalibration
Through Kriyas the subtle body releases dense karmic patterns which cannot be pierced otherwise through techniques which operate on will. The involuntary yoga that happens through grace of shaktipat not only activates kundalini but also safeguards and navigates the process as one advances and is refined throughout the psyche as well as the subtle body .
The degree of expression varies in intensity from light tremors to ground shaking earthquakes, depending upon the blocks in the subtle naadi network and the karmic accounts of a person who is being infused with prana kundalini. The refinement proceeds from strong movements towards stillness of samadhi, and from that perspective the whole awakening process is a grand kriya.
The only way to progress through is to allow full expression and let go to whatever is happening as an observer. Any resistance or need to control will hinder the natural process and make the kriyas more violent and even cause damage. The speed of advancement depends on ones quality of surrender.
As you fully surrender to the shaking it may get amplified initially, though there will be a depth in awareness of the uninhabited region of the body. The shaking/ jerking / asana/ mudra/ bandha is trying to exfoliate and open up that region to sustain more flow. When there is increased flux of awareness enabled, the involuntary movement will cease and a more deeper stillness will ensue.
The Kriyas force one to embody the felt sense presence, beingness. It redirect awareness to stop from flowing outwards towards the external world, and pulls it towards the inner void.