The Kriya yoga system consists of a number of levels of pranayama, mantra, and mudra based on techniques intended to rapidly accelerate spiritual development and engender a profound state of tranquility and God-communion.
Kriya yoga is a very powerful way to walk the spiritual path, but at the same time it is a very demanding way. What it demands out of a person is so tremendous.
For a modern educated person today, kriya yoga would be inhuman because it needs that kind of discipline and a certain exactness about everything.
As Yogananda describes Kriya Yoga, “The Kriya Yogi mentally directs his life energy to revolve, upward and downward, around the six spinal centers (medullary, cervical, dorsal, lumbar, sacral, and coccygeal plexuses) which correspond to the twelve astral signs of the zodiac, the symbolic Cosmic Man. One half-minute of revolution of energy around the sensitive spinal cord of man effects subtle progress in his evolution; that half-minute of Kriya equals one year of natural spiritual unfoldment.”