Mula bandha, the root lock, is described in ways that range from the vague to the alarming. Here is a working version: a light, upward engagement of the muscles of the pelvic floor — the same ones you would use to stop yourself from passing water, but a fraction of that effort.
What is, and is not, engaged.
It is the pelvic floor, lifted gently. It is not the buttocks, not the belly gripped hard, not the breath held. If you are clenching the glutes or your face is tightening, you have recruited the wrong things. Let them go and find the smaller, deeper lift underneath.
Find it once, lying down, with everything else relaxed. A small lift, a release. Again. The aim is to learn the muscle in isolation before you ever use it in a pose.
Using it lightly.
In practice, mula bandha is a whisper, not a fist. A light, steady lift held through the standing poses gives the lower back a quiet support and the body a centre to work around. The moment it becomes a grip, it has stopped helping. Less is the skill here.