“Kriya Yoga” has been translated by Yogananda as “union (yoga) with the Infinite through a certain action or rite.”
It is a practice of pranayama, meaning life-force control, in the astral (energy) spine.
Kriya Yoga is an ancient meditative technique revived in modern times by Mahavatar Babaji. It was then brought to international awareness by Paramhansa Yogananda.
According to Yogananda, Kriya Yoga was known to Patanjali, who described it in his Yoga Sutras (2.49): “Liberation can be attained by that pranayama which is accomplished by disjoining the course of inspiration and expiration.”
Kriya Yoga was also known to Krishna, who describes it in the Bhagavad Gita (4:29): “Offering inhaling breath into the outgoing breath, and offering the outgoing breath into the inhaling breath, the yogi neutralizes both these breaths; he thus releases the life force from the heart and brings it under his control.”
In addition, Kriya Yoga was a practice of original Christianity: it was known “to Christ, St. John, St. Paul, and other disciples.” It is referred to in the Holy Bible by St. Paul, who said (I Corinthians 15:31): “Verily, I protest by our rejoicing which I have in Christ, I die daily.” Yogananda explains: By daily withdrawing his bodily life force, he united it by yoga union with the rejoicing (eternal bliss) of the Christ consciousness. In that felicitous state, he was consciously aware of being dead to the delusive sensory world of maya.”
In Yogananda’s Autobiography of a Yogi, chapter 26 is dedicated to the description of Kriya Yoga. You may deepen your understanding of Kriya Yoga through the book, Kriya Yoga, by Jayadev Jaerschky (kindle version).
Kriya Yoga gradually withdraws one’s energy and awareness within, leads to breath control, purifies the “karmic skeleton,” accelerates spiritual development, and awakens kundalini. Thus it leads to God-communion.
Today there are many branches of Kriya Yoga, with slightly different approaches, taught by descendants of Mahavatar Babaji, Lahiri Mahasaya, Sri Yukteswar, and Paramhansa Yogananda.