It's called the Situation Song!!
The rule is very simple. One person will give a situation and the next person has to sing a song which will go well with the given situation!!!
Here's one example for better understanding!
P-1: Leaving someone forever and requesting the other to forget you...
P-2: Bhula dena mujhe hai alvida tujhe, tujhe jeena hai mere bina...
So guys, isn't it easy? So what are we waiting for! Let's start!!