From my understanding, lag is a pretty simple concept. I think some people try and make it more complicated than it needs to be. Lag is the effect of keeping your wrists cocked (not releasing) as long as possible on the down swing prior to impact. This keeps the clubhead behind your hands. The longer you can go through the downswing without releasing, the more lag you have and the more clubhead speed you will have. The key I have recently learned is that you can't release the same time with each club; especially the driver. For those of us who aren't pros, if we release the driver late into the down swing, at or past the ball, as we can do with a short iron, there isn't enough time to get the clubhead around and back square at impact, causing severe slices. The key to lag is knowing when to release with the different club lengths. With a pitching wedge I release the club when my hands are well past the ball, somewhere around my bellybutton. Until a few days ago I thought this was the proper way for every club. From watching a couple of instructional videos I learned that that isn't correct for a driver or other longer clubs. The longer the club, the longer the swing patch and the the longer it takes to get the clubhead back around and squared.
Back to lag specifically, think of your swing as multiple gears with lag being the highest. Your wrists are the higher/fast gears and your shoulders/spine/arms the larger/slower gears. Try this, don't cock your wrists at all and take a backswing only to where the club is parallel to the ground. Then swing through to impact (still zero cocked wrists). Thats gear one. Now, take the club back parallel and cock your wrists to where the club is perpendicular to the ground. Bring the club down keeping your wrists cocked until your hands are near your waistline. Now beging to uncock your wrists while still swinging with gear 1. Your arms travel the same distance, at the same speed but because you had lag and held on to it until the last minute the clubhead actually travels faster from point a to point b then by just using gear 1. That is lag and the key is learning how long you can hold on to it throughout each swing with the different lengths of clubs. So for me, creating lag is simple math, its learning to control it and release at the proper time so I don't slice the ball that is the high level calculus! Hope this helps.
