I am often amazed at the erroneous imagery used by teachers; it's as if they are under pressure to demonstrate their cleverness to insatiable students craving slick and easy answers. It seems to have worked for you and that may be a case of "ignorance is bliss" - no insult intended. But for every person who is happy with the analogy there will be another, like me, who knows full well that fan/auxiliary belts don't work as described and thus such explanation is pointless. You just shouldn't purposely arch your back at all, any curve such be the natural one of your skeleton. The trick is the one being pulled by the teacher claiming this erroneous imagery to be an aide and it's a false one. There is most certainly not equal tension on both sides of any pulley, driving or driven! I could but I'm not going any further with the dismantling of that explanation. I prefer to concentrate on the actual properties of the body and more specifically on the dancer feeling their own body and the result of appropriate exercise. In fact I often say that if you cannot feel your own body how can you expect your senses to feel the movement of someone else's body. Teachers would be much better if they gave students helpful and real exercises to improve their normal posture and core for normal life as well as preparation for the dance.