"Every golfer should understand that golf is a game of balance, rhythm and timing. You have to swing the club. To do that, your body must be in position, beginning with your head, which needs to start behind the ball and stay there all the way through impact."
To add to Jack's thoughts, I feel it's important for me to dispel a common golf myth…
"Keep your Head Down" is NOT the same thing as "Keep your Head Steady." The head may in fact rotate some as you go back. This slight rotation of the head will help you rotate your torso properly, and get yourself loaded, and into a powerful position at the top. Nicklaus famously imparted a slight turning or rotating of his head as he turned back. This allowed him to clear his shoulders and make a solid turn back as he moved up to the top.
Nicklaus once said…
"I played with Sam Snead when I was 16 and watched that little cocking of his head. I thought...if it's good enough for Sam, it's good enough for me."
One of the most important moments in the swing, where keeping that head steady and back really matters, is at impact. Doing so will allow you to fully release the club after impact, which is a sign of fluid, continuous motion from beginning to end.
Many amateurs move laterally, or sway, as they go back. Some do so as they are coming down from the top. And some do so in both instances. An unsteady head is without question a contributing factor to this.
To this day, and many years after his prime, the Golden Bear remains one of the best examples of what proper execution of even the simplest of fundamentals can do for your game.