Also called the Bradford press, the idea here is to use a partial range of motion and keep constant tension on the delts, which is crucial for overall shoulder growth.
Using a barbell, assume the standard overhead press position and bring the bar behind your head, but only to about ear level.
Lift the bar just high enough to clear your head and bring it to the front, about chin level.
There's no locking out at any stage of the movement. Constant tension is key. Use a light enough weight so that each set lasts at least 60 seconds or longer.