Mula bandha (root lock), like asana practice, is intended to purify the pranamayakosha—one of the subtle, five-layered sheaths of the energetic body.
On the pranic level, mula bandha redirects the energy of apana (the downward + outward flow of energy from the body and elimination of waste). When we redirect apana energy upward to join with the natural upward movement of prana toward the crown chakra - sahasrara , there is an increase of brain activity.
On a physical level, mula bandha consists of a contraction, a muscular lifting-up in the floor of the pelvis.
Since the pelvis is the seat of the spine, practicing creates a safe environment for spinal movement
Mula bandha is said to cut through brahma granthi, the energetic knot of our resistance to change and 'fight or flight' reflexes, which lies in between the muladhara (root) and svadhisthana (sacral) chakra.
Creating lightness and fluidity; the body becomes less earthbound and more mobile.
How to :
Focus on contracting the anus and the pelvic floor (similar to Kegel exercises)
Keep the contraction and continue to lengthen the spine and draw the bone inward and upward
Continue to breathe normally (practicing this bandha should not hinder your breathing or pranayama practice)
Exhale when you are ready to release the lock.