Stop Casting and Create more Lag:
Starting your golf downswing with the arms and upper body and thus losing the angle between the lead arm and the club is called casting.
This action reduces the potential for you reach maximum club head speed and also compromises your ability to get ball then turf contact at impact.
In this video, Derek Hooper explains some of the reasons players make this casting error and shows a progression of drills that allow you to correct this move in your own swing.