If you light a fire you will create heat, of course, and that will cause a person to get up and move if you actually lit a fire under someone.
如果你生火, 你当然会产生热量,如果你真的在某人下面生火, 那会导致一个人站起来移动。
2012 ESLPod
The expression " to light a fire under (someone)" means to do or say something to get that person to work, to do something, especially if the person is lazy or unreliable.
" to light a fire under (someone)" 意思是做或说某事让那个人工作, 做某事,特别是如果这个人懒惰或不可靠。