The King of Lag: Gary Woodland Swing Analysis 

So close to the right explanation. For nearly every golf coach (or at least those on YT) lag is to do with wrist angles which as the video hints at but does not explain fully is fundamentally about inertia at the transition between backswing and downswing. That should lead to the obvious questions which are what is the inertia in the swing and how is it created. The scientific definition of Inertia is the tendency of objects in motion to stay in motion and objects at rest to stay at rest, unless a force causes its speed or direction to change. What that means in the context of a golf swing is that the club head will continue to move in the direction imparted by the backswing until enough force is applied to the club grip end in the downswing to stop and then reverse that momentum. The critical point is that we do not create lag by changing wrist angles, the change in wrist angles is a consequence of the lag. The inertia definition also shows 2 ways to increase lag: swing harder in the backswing so that the club head has more momentum when you get to transition (the Bryson method) or be gentler in transition to the point of having a pause at the top of the backswing (Hideki method) so it takes longer to generate a force in the downswing the overrides the momentum of the club head in the backswing direction. Personally I prefer the second method. Two final and important points: if you grip the club hard enough to strangle it you will also severely strangle the creation of lag, grip it lightly and let inertia create the wrist angles for you; and if you have limited wrist movement because of injuries be gentle!

Nice explanation.