Leading with the hands (or the butt end of the club) ensures a forward leaning shaft and ball-first contact--"hitting down" on the ball with an iron (i.e., hitting the little ball before the big ball). Many term this compressing the ball or "pinching" it off the ground. It is said that this downward strike with a later release sends more energy into the ball (because of the physics of a lever or double-pendulum action), thus providing greater distance and a lower, piercing ball flight. It's also true that releasing later increases the chance of pushing the ball out to the right, because the club face may not have closed in time. Body and forearm rotation close the club face when lagging the club.
First, it's fairly obvious that most professional and low-handicap golfers maintain this angle. You can see it in slow motion video of a great iron swings. Consider Rory McIlroy.
He retains his wrist angles or "keeps the box" (the "L" shape of his left arm to the shaft) well into the down swing; the hands only release the club once they've passed the ball. Notice how his left shoulder points behind the ball at the top. Could this coil be a key to maintaining lag without holding the wrists?
Another way this is often analogized is that one pulls the butt end of the golf club straight down as if pulling on a chain or rope. Sergio Garcia says this is a swing thought for him, and he has massive lag. Others teach that the butt end of the club comes in as if trying to hit something with it by the left thigh or left hip (very similar to Els' "keep the box" analogy).
Some proclaim that one should actively or consciously maintain the wrist angle established by the cocking of the left wrist during the backswing (i.e., 90 degrees formed by the shaft and the left arm). Some teach to hold the angle created with the right wrist (as this wrist shouldn't straighten until well after impact).
Other professionals state that lag is retained passively, automatically, and unconsciously by doing various other things correctly (like McIlroy's 90-degree shoulder turn above), to especially include starting the downswing with a lower body move or pivot (i.e., weight shift back to the left and simultaneous hip rotation). They state the arms remain passive and the angle is retained. Hank Haney states that lag is caused by the transition (i.e., the downswing starting with the lower body as the arms are still finishing the backswing). However, I've seen that some professional golfers appear to have a slight pause at the top and still retain lag, so is this really true? McIlory doesn't. He definitely has that blending between backswing and downswing during his transition.
The author referenced above recommends the "pump-pump-go" drill to sequence it all correctly. Here's another example. But even the last video instructor recommends "holding the angle." Herman Williams has another interesting drill that claims to help eliminate the cast from top to bottom of the downswing; he seems to imply an active holding of the wrist angle.
An important point is that a golfer must never attempt to "hold onto the left wrist angle" by artificially stiffening the left/right wrists - in an attempt to delay the release point. The wrists must always be very relaxed during the entire downswing.
[From How to maximize wrist lag]
I've noticed in my own swing a tendency to cast about midway down. I've done it now for about year, so it has become part of my muscle memory. My hands seem to be doing it (i.e., pushing out with the right hand while pulling in with the left hand). I don't know if there's something else I'm doing in my swing that's causing my hands to cast, but it definitely feels like an active process. Maybe it's an unconscious effort to square up the club face; I'm not sure.
I suppose I'm wondering whether learning the feel of lag can't be an active process. In other words: Is it wrong to consciously address my casting problem with an active holding of the wrist angle to learn what "correct" feels like? If I don't then how will I overcome the muscle memory of casting? Why not hold the lag using my left wrist (or right wrist or both)? Yes, I know this produces some amount of tension in the forearms. I have seen some success with distance hitting this way, but I'm not sure that it's the right feel. Obviously, what feels right to me is casting, which is obviously WRONG. I also have hit some really crappy shots holding the angle, and my scores haven't really improved any yet. It definitely seems like there should be a way to retain that lag without having to artificially induce it. The jury for me is still out.
How do you create lag and maintain it? Think of your left arm as one lever and the shaft as another. Going back, hinge your left wrist to create a sharp angle between the two levers. Then, as you swing down, maintain that angle for as long as you can. When you release it, the clubhead will speed up.
Many amateurs release the lag too soon by unhinging their wrists. A drill to prevent this is to swing back with your left arm only, and as you start down, hook a finger from your right hand around the shaft and resist the unhinging of your wrist. Groove this feel. In a normal swing, your wrists unhinge naturally at the bottom of the arc.
Pretend that there is something near your left hip and you want to hit it with the grip end of the club - not with the club head.
Often golfers mistake understanding lag by solely looking at the angle between the left arm and clubshaft, but it is incorrect. The left wrist will be uncocking at this point in the swing as the clubhead accelerates and begins to square. However, the angle formed by the right wrist and clubshaft continues to be maintained and is critical, especially for Rotary Hitters, as this is what allows you to control the power of your shot and the angle of the clubface at impact. Improving this angle will go a long way to helping you become a better golfer.
How can this be fixed? Well, there needs to be an understanding that the clubhead will always follow whatever the other end of the club does. So, the grip actually should lead or direct the club towards the ball. Once the golf club reaches the "set" position at the top of the backswing (a right angle between the left hand and the shaft of the club) that position should stay the same until the hands and the grip of the club are about even with the golf ball. At that point the hands will begin to unhinge and release down into the golf ball.
To increase your clubhead lag, swing down from the inside, and pull the butt of the club toward your left thigh as your shoulders and hips unwind. Pulling the grip toward your thigh rotates your left forearm and prevents you from making an early release. It also gets the driver back in front of you so you don't have to flip your hands through. The lag releases naturally just before impact, creating a powerful strike.
To do this the legs start by shifting both knees and ankles targetward. It is true that the head stays behind the ball, but the core of the body shifts and clears past the ball rather dramatically. This assists the hands and arms in making their initial drop to the waist high area while wrists are still fully hinged. At this stage the right wrist should still be bent back fully but the palm is now facing the ground not the sky. With the palm facing down this insures the clubface is also facing down or toward the ball and requires no scooping or manipulation to be squared up in time for the hit.
Simply continue to drive the heel of the right hand through the ball as legs and hips clear or rotate through to the finish. The wrists will not fully unhinge until after contact with the ball as the trailing right arm continues to straighten past impact.
I view Ben Hogan and Lee Trevino as perhaps the best ball strikers ever. They both had maximum delay of the right side or “lag,” as it is called today. This allowed them maximum distance and accuracy. In this type of swing, the lower left side of the body moves forward as the club is still going back. This swing is created by a coil of muscles in the left side of the back. Thus, the lower left side responds to this torque in the back and moves forward before the backswing is completed. This multiplies the backswing torque, and adds to the lag in the swing...When this open left shoulder turns away from the less-open left hip, an immediate coiling in the left side of the back occurs. The hips will react to this coiling and move toward the target before the club goes all the way back. Because of this, the right side of the body will be subjugated until the moment before impact. Then when the right side does participate, it won’t interfere with the accuracy of the shot, but it will create more acceleration, culminating in a full finish.
Weight is constantly shifting in the golf swing, from even before the club is taken back at all. To create lag the secret is to make sure that the weight is being properly shifted FORWARD just before the club reaches its position at the top of the backswing. As this occurs a recentering of the center of the body will occur and the hands will automatically LAG behind the body.
THIS IS LAG! The hips shift the hips rotate, the hands FOLLOW or LAG behind. The DOG wags the TAIL. In a great golf swing the hands follow the movements of the body and have no choice but to delay their hit.
CONSCIOUSLY thinking about holding the release, delaying the hands, WHEN MAKING A GOLF SWING is the quickest way to destroy your swing. When you practice, sure, focus on where your hands need to be in the slot on the way back down using a pump drill or some variant of it. But when you are making an actual downswing it has to be a complete abandonment of positions, it is giving up control to get control, it is swinging the club from the body, it is creating lag.