"Keep your core tight."
"Brace your core around your spine."
"Suck your belly into your spine."
"Draw your core in toward your spine."
In my previous article on the who, what, why, and how of using belts I mentioned that you want to brace your abs OUT against the belt. And I had someone ask me what that meant, as it seems a little counterintuitive considering many other cues we usually give and hear.
When it comes to body weight movements, planks and push ups, the way we think to brace the core is right along the lines of the above cues. But when we start lifting heavy weights, we have to think about creating even more tension in the core, and we do this through increasing intra abdominal pressure.
The fancy term for this is the Valsalva Maneuver. From the online Merriam Webster Dictionary:
: a forceful attempt at expiration when the airway is closed at some point; especially : a conscious attempt made while holding the nostrils closed and keeping the mouth shut (as for the purpose of adjusting middle ear pressure) —called also Valsalva
So you've probably done the Valsalva maneuver in an airplane or while scuba diving. But in the gym, the cues we use are "Take a big breath and get tight around it."
You would never be cured to do this during handstands or planks. But I hope you've heard this for deadlifts, squats, and Olympic lifts. The reason this is so effective is this chest full of air give our core muscles something to brace around.
What the belt does is allow this process to be even more effective.
I'll usually use the cue of "breath into the belt" or "breath against the belt." So you're bringing air deep into the lungs, the belt is "in the way" and through this tension between your Valsalva breathing and the belt you get extra core tightness.
Just as with the belt, I don't use the Valsalva maneuver with newer athletes. I'm not going to be taking someone to rep maxes within their first couple of months in the gym, so I generally want them tight and breathing intuitively. When they start asking me about how they should be breathing during a lift, and they almost always do a few months in, that's when I explain most of this.