It only works with heavy doors, most doors are too light (to save money) and so the air pocket cushions the door as it closes and it doesn’t have enough force to close.
Usually, you can only do it with front doors since they’re heavier, however, most people need to lock their doors with a key eliminating the benefit of the behind-the-back flick. It works in this instance because the door has an auto deadlock which means closed is locked anyway.
But with a heavy door I especially don’t want to be slamming it onto my fingers! At least stage doors shouldn’t have air pockets, since the rooms are all open for lighting and filming.
With a smooth enough hinge and little air resistance, you can easily keep your hand moving faster than the door after release to avoid squished digits.
It’s only when the system has extra resistance that you need to give it a good flick before letting go and it’s harder to get your hand back out. You certainly wouldn’t do it with just any door but with certain setups once you’ve gone through enough times it’s just the natural progression of efficiency (and, let’s be honest, a little bit of coolness)