Let’s think a little outside the ‘swimming box’ for a moment and I want you to imagine yourself hitting a tennis ball like Roger Federer.
Before he accelerates through the stroke to hit the ball he opens up his hips to set himself up. Then as he starts his stroke to hit the ball he accelerates his arm movement using his hip rotation to help generate more power in his tennis stroke. If his arm simply moves together with the hips at the same speed as his body rotation he will generate very little power at the point of impact with the ball.
Think of this with your swimming - as your arm goes through the water it must move faster than your body rotation in order to help you accelerate forward and generate power. So think of these as two separate components
1. Body Rotation is your leverage
2. Arm Movement accelerating through the stroke
Your body rotation opens up so your arm can power through the stroke like Federer hitting a forehand winner! Leverage!