Bandhas are physio-psychological locks. They are contractions of specific muscles of the body. The word 'mudra' means 'to hold, tighten or lock'. Their purpose is to lock energy in particular areas of the body and re-direct it for spiritual awakening.
They induce relaxation - physical, emotional and mental, balance the nervous system, slow the metabolism and heart rate, and may reduce blood pressure. They help us to awaken dormant faculties in the brain and in the mind.
We strongly advise you to learn bandhas from a competent yoga teacher.