Kriyas ( pronounced kree-yuhz) are automatic movements, vocalizations or actions that are part of the process of Kundalini clearing the karma held in the energy body. They may take almost infinite forms, from a simple urge to put the body into yoga postures, even if you have never studied yoga- to elaborate songs, chanting and dances, possibly in a language you are unfamiliar with.
Yoga postures, twitching and shaking are the most common types of kriyas. Kurt Keutzer's FAQ contains a simple analogy of what is actually happening: Imagine a garden hose, coiled up and kinked. Turn the water on (Kundalini energy) and the hose tends to thrash around until the water pressure causes it to straighten enough for the water to flow easily. Kriyas most often occur when the body's instinctive guidance is in the process of moving a blockage to the flow of spiritual life energy.
Most kriyas are simple tremors or twitches as tension held in the body, releases. However, they can be quite complex movements, breathing patterns, (pranayama) vocalizations, visions,... the body will do whatever it needs to do, to facilitate the blockage being cleared, and it has all the wisdom of the Collective consciousness, to draw upon. So, a Kriya may take the form of an African Dance, a Sanskrit song, an Aboriginal drum chant, or anything. Energy clearing techniques that have been used by different cultures for thousands of years, tend to have a higher resonance in the Collective, and the body-mind guidance will make use of that unconscious information, bring it forth.
That is why people who may never have studied Hatha Yoga, find their bodies wanting to assume Yoga postures, or why people who do not speak a foreign language may find themselves spontaneously chanting in Sanskrit.
One type of kriya is spontaneously sounding out long notes that make the body vibrate. The wordless vocalizations are sometimes called "toning", but it is clearer to think of them as "tuning", like tuning a piano so the sound is harmonious. Similarly, the ringing in your ears that you may get, is an "OM" that is made by the body being adjusted in vibrational frequency. Crystal bowls and Tibetan singing bowls are designed to have a similar effect, of using sound vibration to effect the physical and energy bodies. In a way, it is similar to the sonic toothbrushes or contact lens cleaners that are sold. The vibration breaks up the blockages so they can be swept away by the energy flow.
The important thing about kriyas, is to surrender to them. Simply relax and allow the body to do whatever it needs to do... so long as your intuitive impulses do not harm yourself or another. Do not mistake negative impulses for kriyas!
Many yogas are based on repeating what some awakened person did spontaneously. A teacher who gets some spontaneous kriya and finds value in it, may teach others to do the kriya deliberately. Kriya Yoga, brought to the west by Yogananda, and taught by the Self Realization Fellowship is based on a simple but powerful kriya that is essentially the same as Kegel exercises that are sometimes taught to pregnant women, or others suffering from urinary incontinence.
Kegel exercises are tightening and relaxing the muscle of the perineum, sometimes called the PC muscle or pelvic floor muscle. It is the band of muscle that wraps around the base of the pelvis, front to back and is responsible for controlling the flow of urine, amongst other things. SRF Gurus advise one only undertaking regular use of this exercise with an experienced guide; the energy brought up by this act is powerful and may result in some discomfort or psychosis caused by karmic issues being triggered faster then the individual's capacity to resolve them.
Some people find kriyas to be a source of embarrassment, especially if they come unexpectedly in public. Issues of embarrassment are caused by ego, and the ideal is to allow the processing and let Goddess handle the reactions. Surrender. However, if you are not ready to do that, sometimes one can negotiate with the Kundalini Shakti, and ask that kriyas only occur in private... but in exchange, agree to spend some private time every day, allowing the body to have kriyas as it will.