One of the biggest mistakes that amateur golfers make is to keep their head down.
If you had a lesson with a PGA Golf Professional you would never hear them saying 'keep your head down' as this is incorrect during your golf swing. If we look at today’s leading tour players, we see them allowing their head to drop slightly on their down swing as they compress themselves towards the ground and then they lift their head during impact as they explode upwards through the shot, allowing their head to rotate towards the target and rise during their follow through.
The next time you are at the range or practice ground, rather than working on keeping your head down, work on maintaining your head position in regards to a left to right directional movement. Keep your head static and do not allow it to move right or left. However, do not keep it down, especially on your follow through as this restricts your movement, power and therefore speed and as a result, you will reduce your shot distance and shot accuracy.
As you swing through impact, allow your head to rotate towards the target so that you can follow the flight of the ball with your eyes and as your right upper arm (for right handed golfers) touches the right side of your chin/face, allow your head to turn towards the target and lift upwards. This will allow your upper body to fully rotate left of target and allow your body to turn through the shot with your arms. This will create maximum club head speed but also will allow you to swing the club head down the target line and hit straight shots. If your head does not rotate, your upper body will be restricted from fully rotating left on your follow through and the club head will be slowed down due to this producing a shorter shot.
Keeping your head down will also result in your arms overtaking your body movement on your follow through and as a result, you will pull your club head away from the target line and on to the inside and this will now create directional issues with your golf shot as if you close the club face you will hit the ball left and if you try to keep your club face aiming at the target, the ball will now curve left to right and produce a slice.
For your most accurate, powerful and longest golf shots, rather than keeping your head down, work on keeping your head static from a left to right view point and allow your head to rotate and rise on your follow through to allow your upper body to move correctly and most efficiently.