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Tell them to imagine breaking down a door with the left shoulder, to feel as if hitting the ball with the back of the left shoulder.

Edz,
So how does this help someone create grounds forces?.
If someone tried to feel if they are hitting the back of the ball with their left shoulder.
Wouldn't this promote an over the top move?
Aren't you teaching them to fire their upper body first? Instead of engaging the lower body first.
Would this also cause a lateral slide as well?
Trying to put weight on your left foot doesn't creating ground forces.
Get some pressure plates or forces plates try it and see for yourself.