We are probably saying the same thing. When people kick the air they tend to keep their hips in the same place and just move the leg. This creates a habit of not using the forward momentum of the body in the kick. By driving the hip forward the upper body "leans" back- so to my mind it's the hip going forward not the body leaning back that adds power. Same thing on a knee you bring the hip into it by arching the back-if you just arch the back you lose power.
Hope that makes sense.
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Duane