Ive never tried behind the shoulder presses but if its about side delts, i'd rather hit them with cables lat raises ngl. But yea, this is a cool exercise to try out this weekend.
Are you still hanging? That's all the shoulder health you need to try such exercise
Ease into it with the empty bar, dont go all the way down, stop midway through your head
Find the comfort zone for your shoulders, go wider than a normal press
Seriously, my shoulders literally exploded after a couple months of behind the neck pressing, sometimes I superset it with db laterals
Warm up well before you press
After you get comfortable with it and begin adding weights, if you pull the bar from a squat rack be sure to push press the first rep then continue normally