Mula Bandha

"Mūla Bandha is a Sanskrit (मूल बंध) compound term: Mūla denotes "root", "base", "beginning", "foundation","origin or cause", "basis", "source"; Bandha denotes "bondage", "fetter", "posture",[2] "joining together", "catching hold of"....Mūla Bandha is the principal, key and primary Bandha of the Yogic traditions. Mūla Bandha is endemic to all safe, grounded workings of bodymind disciplines. This Bandha in and of itself conditions the Muladhara Chakra, simultaneously keening, rooting and engaging the systemic plethora of processes that constitute bodymind and with diligence resolving them in discipline and accord. Mūla Bandha should be held as a restraint only after kumbhaka, which in this instance is where the breath is expressed in its entirety and held outside the body. Iyengar (1976: p. 435) likens the functionality of the Bandha and especially Mūla Bandha to "safety-valves which should be kept shut during the practice of kumbhakas"" (Wikipedia)

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Mula Bandha in action
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Bandhas for Beginners: Intro to Yoga's Interior Locks
Are you Engaging your Mula Bandha Correctly?