i was fortunate to have the opportunity to go through a gunsite defensice pistol session many years ago. the five step was taught in that session but i don't remember being told to place the off hand behind my head. to the best of my memory, i believe the instructor(i don't remember who but Lou was there, not teaching this course) taught us to place the off hand over our abdomen keeping it clear of the muzzle. in this stance, the off hand can be used for defensive purposes if needed, i.e. a good left hook once the shot was fired. i still practice this technique today.