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。

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。