iNhale deeply.

Take a deep breath into your belly and hold that breath throughout the first rep. Only take shallow, "top off" breaths at the top of each rep or two. Pressing this air down and out will help you stay rigid and transfer energy from your legs into the press.
Deep Breath + Top Offs
As Marissa brings the bar out, she takes big air in before the descent. She keeps the chest high, keeps that air in, presses and holds her air until she's back at the top of the lift. If it's a lighter set and you can move the bar pretty quickly, it would be fine for you to do multiple reps within the same breath, but what we definitely don't want is Marissa to take a big air in and then either breathe out with the bar on her chest or breathe out as she presses.
Juggernaut
BREATHE AND BRACE BEFORE UNRACKING
BREATHE AND BRACE BEFORE UNRACKING
I can never get as much air into my belly when I'm under load as I can without, so I get a huge belly breath and brace it down before I unrack the bar... That entire first rep is all on the same breath. I will do this for anywhere between one to three reps, depending on how many I'm going for. If at that point I need to take in more air, I will get my air at the top of the rep. So let's say I take my breath, I unrack the bar, I get three reps, and I need more oxygen. At lockout, I will rebreathe (take a couple little breaths), re-brace, huge belly breath, rebrace and then drop in for reps... then I just continue this pattern for as long as needed. Never EVER breathe when the barbell is in motion.
Neversate
Taking my air before I unrack the bar and not letting it go... I find with the weight pushing down on me, I'm not able to get as big a chest or as much air as I can before I unrack the bar. So holding my breath a little bit longer has really allowed me to stay a lot tighter out of the rack and has also allowed me to be a lot more consistent.
Calgary Barbell
Hold your breath throughout the duration of the rep. Take a deep, diaphragmatic breath before you start the descent, and release it once the bar is locked out, or at least nearing lockout. This will help you maintain tension and stability.
Stronger By Science
Try to not let all your air out between each rep, simply try to top each breath up at lock out. When you do take a full breath, reset. Make sure to keep your shoulders in place and keep your legs engaged, so as not to fall out of position.
MPS Training
Breathe before you unrack the bar, and not after... Breathing before you unrack the bar allows you to fully breathe and expand through the midsection without worrying about the weight in your hands. I’m sure we can all agree on the fact that it’s easier to breathe into your belly without 400 pounds in your hands...Proper breathing leads to being able to brace the midsection more effectively. The greater the bracing effect you create through your midsection, the more force that will transfer from the lower body into the bar.
Breathing before you unrack the bar allows you to fully breathe and expand through the midsection without worrying about the weight in your hands. Proper breathing leads to being able to brace the midsection more effectively. The greater the bracing effect you create through your midsection, the more force that will transfer from the lower body into the bar.
Breathing after you load your body with weight really limits your ability to expand and brace under load. Instead breathe deeply and fully before taking the weight out of the rack and if you need to reset, breathe at the top of the rep.
Big Benchas
Stay Rigid + Transfer Energy
BREATHE BEFORE UNRACK
BREATHE BEFORE UNRACK
Calgary Barbell
You should still be able to make a large belly when in this extended position. We have to keep in mind that in a bench press position, because you're so extended, all that air is going to the anterior portion of our midsection. So we should still be able to create that large belly draw down on the rib cage and produce [pressure in your abdomen].
Kabuki Strength
Take a deep breath into your gut, pressing the air out against your belt (if you have one) and... puff your chest out... to expand your ribcage as much as possible.
Jeff Nippard
This decreases the range of motion within the benchpress and also puts the musculature of the shoulder girdle at a better position to create force.
Barbell Medicine
Take a deep breath into your stomach, just as you would for a squat or deadlift. This will help keep your torso rigid to transfer force from your hips to the bar when you use leg drive.
Stronger By Science