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.