Imagine you’re sitting upright and have a pen oriented vertically between your shoulder blades. Now, pull your shoulder blades back so that you’re pinching the pen in place (see photos).
This is what you want to do when benching as well - only lying down. Retract the shoulder blades before you unrack the bar, re-retract them after the unrack as well in case you lose some shoulder positioning during the process of unracking, and then keep them retracted throughout the set. Additionally, if needed, re-retract them between reps if they get out of position.
Retracting the shoulder blades creates more stability and puts us in a stronger, safer, and more efficient position from which to bench. Not having your shoulders retracted is the benching equivalent of squatting while standing on your toes - just plain dumb. Retracting your shoulders also has the added bonus of shaving off just a bit of distance that the bar has to travel (just like squatting while flat-footed means you don’t squat as far as you would if you stupidly squatted while balanced up on your toes).
As always, we hope this helps you get stronger and live better.
