介詞on在這里的意思是：
（身體重量）由（某部位）支撐
You use on to say what part of your body is supporting your weight.
He continued to lie on his back and look at clouds...
他仍舊仰臥著,望著天上的云彩.
He raised himself on his elbows,squinting into the sun...
他用胳膊肘支起身子,瞇著眼睛看了看太陽.
She was on her hands and knees in the bathroom.
她趴在浴室的地上.
祝你開心如意!