Gripping the club too tightly, especially with your strong hand, can make your swing cut across the ball instead of connecting with it directly.
A light grip can actually produce better results than a tight grip, because unnecessary muscle tension slows down the speed of your swing [source: LearnAboutGolf.com].
On the left hand, all the pressure on the grip should be coming from the last three fingers. Neither the forefinger nor the thumb of the left hand should put any pressure on the club.
With both hands, pressure on the club should come from the pads of the hand and the fingers, but never the palms.