What is Life-force or Prana?
Life-force, or Prana as Yogis call it, is the energy of Awareness. Energy and Awareness are the two sides of the same coin which is the basis of the whole universe. When all matter is destroyed, energy is what remains. Energy is the original form of all matter. Awareness is the other original since no dead matter nor life-less energy could have produced Awareness (as the materialists and atheists would have us believe), unless Awareness was already inherent. An important aspect of any Yoga system is to balance the negative and the positive currents of this energy. One of the best methods to balance these currents is called Alternate Nostril Breathing (nadi shodhana).
1. Preparing the body
To promote health and gain a certain control over body functions, some practice Hatha Yoga. Others, who are not that flexible or are restricted in time, are content with a 15 minute program of easier exercises consisting of stretching and bending designed by Yogananda for maximum result.
2. Preparing the mind
Kriya Yoga includes and combines many useful yoga techniques. Like Raja Yoga, Kriya teaches the laws of general conduct (yama and niyama), including harmlessness, truthfulness, non-stealing, etc. Kriya Yoga promotes the study of metaphysical principles, physical and mental health, cleanliness and purity.
For the purpose of practicing Meditation, Mantras are introduced. Lessons or initiation teach the seeker how to use mantras like So-ham, Ham-sa, Hong-so and OM.
4. Life-force control (pranayama)
At this point the difference between Kriya Yoga and other yoga systems becomes quite obvious. Kriya pranayama is not so focused on increasing the time of breath retention, but is used to magnetize the spine and direct life-force to the brain so as to refine the brain and nervous system.
5. Initiation and Shaktipat (transfer of energy)
The seeker is initiated in the proper use of Kriya pranayama. When the seeker is ready, a transfer of energy might occur either from the outside or from within. To experience Kundalini (energy) on its way up the spine is an event powerful enough to change the way we think and function.
The mudras are added to gain even more control over the flow of energy.
7. Higher Kriyas
For advanced students there are still a few higher Kriya techniques. However, full Self-realization may already be gained by practicing faithfully the Mantras given for regular Meditation.
For anyone interested in Kriya Yoga, one of the most important and inspiring literary works is Yogananda's book, Autobiography of a Yogi.
Further Reading: Self-Realization Articles ~ Six Yoga Systems ~ Yoga Style Glossary