Definition
Tightening the trunk before a lift by breathing into the belly and pushing out against it, so the spine stays stable under load.
Breathe in all the way round the waist, hold it, tighten as if about to take a punch, lift, and breathe out at the top or after the rep. Under a heavy bar the breath is held for the whole rep; on lighter work the client can breathe through.
The brace transfers force from the legs to the bar and protects the spine. Cues that work: "belt without a belt", or a hand on the side of the waist to push against.
Clients with blood pressure conditions or who get dizzy should breathe out through the sticking point instead of holding.