It's Shoulder Tilt...AND...Shoulder Turn

This topic is among golf's best kept secrets.  Why do golfers swing over-the-top or outside-in?  It's because they turn their shoulders on the downswing rather than "tilt" first.  It is the shoulder tilt on the down swing that creates the inside path and angular acceleration, (club head lag).  As soon as you turn your shoulders, on the down swing, the golf club will travel outward.  Being that centrifugal force travels outward, the golf club shaft will loose it's angle and the result is you loose club head speed.

Take a look at this video and drill that will help you create both lag in your down swing and increase your club head speed.