介词on在这里的意思是：
（身体重量）由（某部位）支撑
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...
他仍旧仰卧着,望着天上的云彩.
He raised himself on his elbows,squinting into the sun...
他用胳膊肘支起身子,眯着眼睛看了看太阳.
She was on her hands and knees in the bathroom.
她趴在浴室的地上.
祝你开心如意!