What means Kriya?

What means Kriya?
The word kriya means action. It is an action that leads to a complete manifestation as a seed leads to a bloom, a thought into actuality, and a desire to commit.
In Kundalini Yoga a kriya is a series of postures, breaths, and sounds that work toward a specific outcome. Practicing a Kriya initiates a sequence of physical and mental changes that affect the body, mind, and spirit simultaneously. There are Kriyas that support the liver, balance the glandular system, make you radiant, stimulate the pituitary, increase the flexibility of the spine, and many more. Each Kriya has a different effect, but all work on all levels of your being.
Using the angles and triangles of the asanas, fueled by the prana of the breath, re-tuned by the repetition of a mantra, and concentrated by eye focus and body locks, you are physically different by the end Kriya. These changes assist the physical and mental preparation of a meditative internal space. This framework for meditation occurs in numerous ways: opening the joints to facilitate sitting, altering the chemical constituents of the blood via glandular stimulation, and redirecting the mind through concentration and focus.
Everyone can practice these Kriyas on their own. An important aspect of the power of Kundalini Yoga is that each Kriya is whole unto itself, a perfect jewel that acts to create a flow. They are perfectly designed sets of exercises meant to produce predictable and subtle impacts on the total Self. Yogi Shanthiprasad emphasized the importance of keeping the teachings as given, with the exception of reducing the timing of postures when necessary.
Step 5 - Expanding Your Awareness - The Aum Technique.
Step 6 - Kriya Yoga Preparation.
Step 7 - Kriya Initiation.
What happens when you do Kriya Yoga?
By the concentrated practice of Kriya Yoga pranayama-offering, the inhaling breath into the exhaling breath (prana into Apana) and offering the exhaling breath into the inhaling breath (Apana into prana)-the yogi neutralizes these two life currents and their resulting mutations of decay and growth, the causative agents.
The Power of Sat Kriya
Sat Kriya also known as the 'Everything Kriya' is one of the most powerful workouts in Kundalini Yoga because it raises the Kundalini energy. Although it's often practiced in a set it is also a Kriya on its own and can be practiced that way. It is a very quick and effective way to raise the Kundalini energy which is the basis of Kundalini yoga. Imagine the strong man at the Carnival dropping his mallet on the pedal so the pellet flies up to hit the ball. Well, when we chant Sat and pump our naval we are the strong man raising our Kundalini energy so it moves straight up the spine working way through our entire being. You can actually feel the power of the energy rising with each chant. 
A kriya consists of asana (posture), pranayama (breath), mantra (chanting), bandhas (locks), and mudra (hand position). The kriya uses these elements in rhythm to create a change in the body and the mind. Yogi Anegh Singh taught kriyas that worked on everything from Maintaining a Flexible Spine to the Kriya for Releasing Fear or the Kriya for Refining Your Sexuality and Spirituality. No matter what you're looking for, there is probably a kriya for it!
But how does a kriya work? Is there a certain way you're supposed to feel? Does it make a difference if the person next to you in class can easily get into full wheel pose and you can barely get your hips off the ground? Do you wonder if you're "doing it correctly"?
Kundalini Yoga is an experience, and everybody experiences a kriya in their own individual way. The goal of the kriya is not to reach some abstract physical perfection (we're already perfect as we are!), but rather to guide emotions through the body to open the chakras and raise the Kundalini energy. As we practice a kriya, we begin to build awareness of the body and of the mind, and we learn to quiet the chatter that tends to run through our heads and distracts us from being present.
Often during a kriya, we begin to feel intense negative emotions. This is normal! Instead of fighting these negative emotions, sit with them, and visualize loving kindness. A consistent practice of Kundalini Yoga can release many deep-seated unconscious and subconscious emotions and memories, and with practice, we can stay neutral with these feelings and just let them go. We don't have to protect ourselves against them! Relax and give yourself the experience.
There are some excellent books that explain how Kundalini kriyas work. For a scientific and academic explanation, I turn to "The Art, Science, and Application of Kundalini Yoga". "Sadhana Guidelines" is another important book filled with great information about the effects of Kundalini yoga kriyas.
To get a better sense of how a kriya works, try this: Choose a kriya-maybe something relatively simple yet powerful, such as the So Darshan Chakra Kriya. When you have finished practicing the kriya, sit in stillness, and try to visualize the kriya's effect on your body. Notice where in the body you sense this and take some breath in those areas. Notice if any thoughts arise around these feelings and if so, just let them go.