Get Your Damn Air
Lately I have noticed a few of my new lifters "sipping" air gently before lifts, especially jerks, getting about enough air to fill one infant lung at best.
In order to pressurize the trunk to create maximal rigidity, you need a lot of air—like all of it. You need to fill the lungs, which requires getting your guts out of the way so your diaphragm can contract. If your abdomen doesn't visibly expand as you're taking in a breath, you're not taking in a full breath. Getting nothing but a little rib lift with your breath indicates a shallow, chest breath only that will do little to help with trunk stability.
Before you take in your last big breath, relax your abs a bit so they can expand—your guts have to go somewhere so your diaphragm can move down as it contracts. Keeping your abs locked in tightly when trying to take a deep breath makes as much sense as trying to blow up a balloon while you squeeze it in your fist.
Once you've genuinely filled your lungs, THEN cinch down your trunk circumferentially—abs, back, and everything in between (and crush your guts between your diaphragm and pelvic floor).
Maximal trunk rigidity means maximal force transfer from the legs/hips to the bar and maximal protection for the spine—don't half-ass it.