Gotta agree with the Nelson, behind the neck pressing is not bad for you provided you have a decent background with shoulder stabilizing movements. I did them heavy for a few years and never had a problem, but I also do a lot of overhead moves including heavy snatches. If you can do a snatch then there is no reason you can't do a bnp. On a side note, if you want to develop this move the easiest way to start if you have bad shoulder flexability is to use a cambered bar turned sideways. This puts the bar essentially right over your shoulders and not all the way behind the neck, and gradually over time you should be able to move to a straight bar.