Close the gap and keep lead-leg flex from the top
Craig Hanson's exercise for compression: from the top of the swing, close up the gap with the trail elbow and push slightly downwards while keeping some flex in the lead leg. He says the wider the gap, the more you lose angle and tend to stand up and flip, producing thin shots. If the lead leg straightens too quickly you spin out and lose compression; keeping it bent lets it move further forwards.
Channel: CraigHansonGolf
Watch official source at 11:25