When I was having lessons my pro taught me that in correct start of your downswing you need to push your hips in a straight line 45-degrees left of the target diagonally rather than laterally. He used the video to demonstrate how all top pros move their left hip from five to ten inches diagonally from their setup position to initiate the downswing.

To learn to make this hip movement he put a golf shaft (you could use an umbrella or even a chair) in the ground five inches to the front left of my left hip along this 45-degree line. On the downswing, he had me bump that umbrella with the left hip, keeping your head my head behind the ball. Again he stressed it was important to note how I felt that move (for me its the chest). This is a drill I still use every now and then. It would probably work well with gregs impact drill where you get up against a door/wall