Look at this article by Donald Hutchinson, Jr. from Helium.com:
...by keeping your head still it acts as a pivot point much like how a pendulum swings from a fulcrum. The pendulum passes through the same set of points as long as it swings from the same point and is not disrupted by any other force.
He later offers two simple yet fascinating answer to the question of why golfers are tempted to move their heads:
Pulling your head away before the shot is completed is a common mistake and there are a couple of reasons for this.
One being, trying to hit the ball hard, resulting in a jerking motion or uneven distribution of weight from one leg to the other; causing the head to shift and miss hitting the ball because the arc of the club head has become out of sync with the point the ball lays on.
The second is getting overly anxious about picking the ball up in flight. Golfers want to observe the results of their stoke and lift their heads early in anticipation.
Both of these answers - trying to hit the ball hard and wanting to see the result of your swing immediately - seem to reflect a lack of inner confidence and control on the part of the golfer.
The same is true for speaking - whether you are in a one-on-one conversation or addressing a larger audience.
Often, when we speak, we move our heads a lot. This may be because we are trying so hard to communicate our message - or because we are checking to see how that message impacts our audience. Just like the golfer.
And like the golfer, the speaker who moves his/her head a lot communicates a lack of inner confidence. The effect that this has is to lower the speaker's status.
Sometimes, as a speaker, this is a good thing. If you are the known expert and are trying to meet your audience at their level - lowering status may be just what you need.
And sometimes, it's exactly what you don't want to do.
From the Green Room: If you are trying to raise your status - to impress the audience and show your authority - try keeping your head as still as possible when you speak. This is not to say that you don't look around the room. Rather, keep your movements controlled and purposeful, rather than jerky and random. And always keep your eyes directly focused on your listeners.
For more on keeping your head still, read this post I wrote last year about Patrick Swayze's dancing: http://sarahgershman.blogspot.com/2009/09/great-speech-coach-once-gave-me.html.