This will almost force your lead arm to relax and fold on your forward swing, helping to deliver a square face at impact. One of my favorite drills to combat this is just to take practice swings with your hands split apart.
If the lead arm stays too stiff for too long on your forward swing, this will not only hold the club face open and making the ball slice, but also delays the release of the club face and the speed associated. Having width can add power, but if width is misunderstood to mean lead arm stays straight, this can make it really difficult to allow the lead elbow to relax and fold on the forward swing. Width and tension in a golf swing can easily be confused. If you allow this natural curving to happen as your under arms stay closer to your body throughout your are much more likely to have a square club face that also engages the body in the swing, additionally adding power. What can help you to do this properly is to understand the the golf swing is circular in nature. If on your backswing or your downswing your underarms move away from your body, it is very easy to open the face. One of the great stabilizers of a square club face is to keep your underarms relatively close to your body throughout your golf swing. Your lead hand in particular should appear exactly as it looks when this arm hangs and in most cases that would have the thumb off center and opposing the hand for relaxed support and control. One of the more common misconceptions when it comes to a “weak” grip causing an open club face is golfers thinking their thumbs should go straight down the grip. When making this adjustment, often times it needs to be adjusted more than you think. In other words, if you were a right handed golfer, you would rotate your hands to the right. You can make this adjustment by turning one of both of your hands away from your target. The most common cause of an open club face is an incorrect grip as your hand position will directly reflect in the face. This open club face will give you contact that isn’t square and often feels like a “side swipe.” The most common cause of a slice is an open club face.