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This old David Leadbetter drill
has become a go-to for me.
Our oblique muscles wrap around the sides of our body in a kind of cross fashion. My bad habit is that I tend to dip down towards the ground on the backswing, which robs my ability to stretch these muscles, and costs me power.
Basically you put an exercise band under your left foot and over your right shoulder as you make a backswing. It looks a little goofy, but it gives me a great feel of turning and stretching up towards the sky.