The Behind the head Barbell Press works the front, outer and rear deltoid muscles in the shoulder, as well as the trapezius in the upper back, triceps (upper arm) and serrates anterior (chest).
Set yourself up with the bar on your back and your hands one and a half shoulder widths apart. Take a few steps away from the rack. From here, ensure you have a slight bend in the knee and your core is engaged and tight. Press the bar above your heads achieving straight arms at the top of the movement, before bringing the bar all the way back down to your back.
While bringing the bar back down on every repetition, be careful not to hit the bar into your head or into the back of your neck. You will likely get used to the movement very quickly and learn exactly how to bring it back down.