Another reason why you don't want to go behind your head is, Think about this... what direction are all the muscle fiber's in your lat's leading towards, They don't go straight up and down, so why put the bar behind your head going up and down? Those fibers rap around your back from your spine to your rib cage, on an angle, so having the bar infront of you, leaning back about 2 inch's from a sitting straight postion pulling the bar down to your chest, will cause the greatest muscle contraction.

- Decoder