Lag in a golf swing equates to the delayed release of the club-head to maximize power and efficiency. During the downswing, if the wrists remain hinged and the hands remain ahead of the club-head for a longer period of time, the effect will be similar to the cracking of a whip through contact, with the club-head releasing forcefully through the ball and sending that little dimpled son-of-a-gun into the stratosphere. This is the real secret to achieving more distance.
Here is how to get more lag in your golf swing:
Take the driver out of your bag. Turn the club upside-down so that you are holding the club-head and the grip is facing the ground. Take your normal grip just under the club-head and pretend like the end of your grip is the actual club-head. Take a few swings, and listen for a 'whooshing' noise. This noise occurs at the moment when the club is traveling at the fastest speed. Ideally, we want to hear that noise at the exact moment when the face of the club-head should contact the ball, and not beforehand. Generally speaking, more novice golfers will hear the 'whoosh' too soon.
If the 'whoosh' noise occurs too soon, like around the time when the shaft of the club is traveling past your right knee, or somewhere thereabouts, then you are releasing the club too soon (or "casting" the club as it's commonly referred to). This over the top move is so common because the average golfer has a tendency to grip the club too hard at the top of the swing and try to rush it down with the arms and hands rather than letting it come down naturally as it should.
So, take two tees and place one right where the ball would normally be, and put the other about a foot ahead of it. Practice the 'whooshing' drill and try to get the 'whoosh' to occur right between the tees. When this happens the hips have time to clear out of the way and the body weight has time to shift over to the left side of the body (for right handers), which guides the club head through the contact area much faster.
Remember, it is not the arms and hands that create power in the golf swing. Rather it is the uncoiling motion of the upper body coupled with a powerful weight transfer to the left side in which the right hip fires through as it rotates. When this happens the proper lag is achieved by the body motion leading the arms and club down and through, instead of the opposite.
Other drills to help incorporate more lag in your golf swing include; practicing golf swings with your left arm only (as it's the right arm/ hand that tend to be the culprit when releasing the club too early - this drill builds up the golf muscles of the left arm and trains the golfer to be more dominant with the left side), and practicing chip shots (as the wrists should never break down or come unhinged during the typical chip shot - this drill teaches the golfer to 'hold on' for longer and delay the release of the club-head).
Good luck and have fun reaping the benefits of added lag!