Here's what a couple of pretty good players think.
Hogan - "THE HIPS INITIATE THE DOWNSWING. Starting them first and moving them correctly -- this one action practically MAKES the downswing. It CREATES early speed."
Nicklaus - "The first move down is the replanting of the left heel, which starts a chain reaction upward through the body--it pulls the left knee toward the target, which pulls the right knee toward the target, which pulls the hips around and in turn the torso, shoulders, arms, hands, and club."