You need to hang completely free from a chin-up bar, legs straight down and your body relaxed. Then slowly build up the muscle tension and tighten completely up
What do you mean by “tighten completely up”? Do you mean that you should hold your breath and tighten your core as you do when, say, doing squats?
I tighten from my hips up to my hands - the only part that does not tighten are the legs (the butt does tighten). Again, take a few seconds to build up the tension - it’s a matter of trying it some and getting good tension without turning red and busting out your veins - it’s not that intense. I do not hold my breath, but I am only holding final tension no more than 3 seconds. Breath is shallow and the core is rock solid. The release is where is all happens - build to tension - then relax.
Once you release (relax)you should feel your hips drop slightly from your upper body. Let the air rush out of your lungs at the release - no need to push it, just “let” it out as fast as possible. The hardest part of it is really in the release stage - learning how to actually and completely relax the back without letting go of the bar!