Your shoulder is made up of three bones: the humerus (upper arm), clavicle (collar bone), and scapula (shoulder blade). The humerus connects to the scapula via a ball and socket joint called the glenohumeral joint. So any articulation of your upper arm will involve your scapula to some degree.
The funny thing about the scapula is that it’s not directly attached to your skeleton. It is held in place by muscles in your back. If these muscles are loosey-goosey when you try to push something heavy over your head, the scapula will sag and you will be leaking energy out of your shoulders. The heavy thing is likely to bonk you on the head. If the heavy thing is your body, and you are doing a handstand, you might bonk your head on the ground.
Before you press overhead you should give yourself something solid to push from by engaging your shoulder blades. But how can this be accomplished? Most of us are very “connected” to our front sides, probably from years of navel gazing and minute examination of our abs in the bathroom mirror. How are you supposed to connect to some weird bone in your back that you can’t even see?
Here’s a drill that I will freely admit I stole from Pavel. Stand with your back against the wall with your knees bent. Walk your shoulder blades up the wall in a R-L-R-L pattern until your knees are straight. Strive to move each of your scaps independently and really reach with each. Now work your way back down. Repeat!