mula bandha

This is the root lock. It is located at the base of your spinal column in the perineal muscle. This is a deep muscle in the region between your anus and genitals. Another way to think about it is by imagining you have to stop the flow of urine mid-stream while going to the toilet. The contracting of the perineal muscle is subtle, you do not need to contract your buttocks, but rather focus on contracting only the perineal muscle.
Mula Bandha is a key lock in stabilizing the pelvic region during your practice.