When engaging in a complex compound movement with a heavy (relative to one's 1RM) load, consciously activating the "core" (the musculature of the trunk) is an integral element toward progression through the target range of motion.
Engagement of the core may seem like a vague term but in actuality it is a simple, easy to perform process. To properly engage one's core, activation of the transverse abdomini is pivotal; simply focus on: 1) inhaling deeply to create a stomach vacuum during the eccentric portion of the movement's range of motion and then 2) contract the musculature of the abdomen by exhaling while performing the concentric movement toward completion of range of motion.
Try these 2 simple steps the next time you are performing a bench press, deadlift or squat with 75 percent or more of your one rep max and notice the relatively large increase in strength it provides!
Nice....it's almost automatic unless you're teaching it for the first time. Breath in, Breath out. Why you're posting this bullshit on here where 95% of the people on here train is beyond me. but have at it.