Short Thumb Grip
Use a 'short-thumb' grip for more clubhead control and speed.
One aspect of the grip I rarely hear discussed is the 'short-thumb' and 'long-thumb' styles of holding the club. They merit attention because they have a subtle effect on other features of your left-hand grip.
Ultimately this determines how much hand action you use during the swing, and how well you control the club at the top of the backswing.
The long thumb I'm demonstrating for you here (below left) is rarely used by better players. When you extend your left thumb far down the shaft, it causes the handle (grip) to shift towards the palm of your left hand. That's not good; a palm grip restricts hand action and thus your ability to generate clubhead speed.
Moreover, you can't control the club very well during the late stages of the backswing. The club tends to bounce on you at the top. The long thumb sometimes gives the sensation of giving you more control, when in fact it provides less. I rarely lengthen a player's thumb position.
I much prefer the short-thumb position I'm demonstrating (below right). Actually, I like to call it a neutral thumb, because that's where it should be. You can see how the handle now rests more towards my finger tips. If I opened my hand you'd see the club runs along the callus pads at the base of my fingers, but not beyond them into the palm.
The short thumb allows my left wrist to hinge fully on the back-swing, and because my fingers are in position to control the club, I'm capable of generating all the clubhead speed I need coming down. In other words, it allows you to enjoy your full speed potential.
Experiment with the two thumb positions next time you practise, and see if what I'm saying isn't so. This is one fundamental you simply cannot afford to overlook.