More slide rule text book training advice, never taking into the consideration that all men are not created equal Might as well have a dozen model skeletons for display. Also not considering the muscle inserts and bone length for each of us. This gentleman is selling a product, himself and his web site.
The Press Behind The Neck, like upright rows, is not meant for everyone, as are the full squat and DL Some body mechanics, and bone structure, are just not adaptable to any of these movements. These two movements, the PBN and up-rights, are two of my favorite exercises and have been doing them for years.
With the Press Behind The Neck, most guys are just not flexible enough at first. Suggest a training dowel (or exercise bar) at first....just to get the feel of the movement and to warm up the area. This is a learning exercise, so add weight slowly until getting use to these. Also just lower the bar to the around the base of the neck or upper traps. No need going any lower. Don't try to advance the weight too fast or get stuck doing more weight than you can handle. Don't lock out at the top, if wanting to use the time under tension (TUT) BB'ing method. Never get in the habit of going to failure in any of the above exercises. Can cause more problems, in the long run, than they are worth.
If after a few weeks this exercise does not feel right for you, than drop it and on to something else. Same can be said for up right rows.
Good Luck.