I'm going to start with something very controversial...I believe that being able to maintain a correct plank position, is one of the most important form analysis a trainer can do. If you can not maintain a plank, you have NO business overloading your body with extra weight in big complex movements like squats, cleans etc.
Why? Because if you cannot master your own bodyweight in the plank, you are asking for injury!!
So what is a correct plank position? Elbows under shoulders, hands and forearms flat to the floor, eyeline your hands, hips and shoulders inline, glutes tight and pushing back on your heels as if you are stretching your Achilles.
Abdominals tightened, so your lower back is supported, but not so much that you are incapable of breathing.
This position, is the preparation for SO many exercises, squats, deadlifts, military press, the list goes on. It may not be an exact replica obviously, but the stacked hips, braced chest, tightened abs and glutes, is the bare minimum you need to do to prep for most lifts.
It is this preparation and being able to breathe through it that will protect you from injury, for the most part, obviously there is ALOT of things that can go wrong when lifting, but being able to hold the plank correctly and breathe through it, will improve your form on all exercises greatly!!