This achievement is for activating an Easter egg in which a guy I presume to be involved with the production of the game pops out of the corner of the screen and the word "Toasty!" is heard. This, of course, is an homage to sound programmer Dan Forden who was the original Toasty man in Mortal Kombat II.
Anyways, using the following setup you should be able to make him appear:
-Start a game using 4 players (any more and it doesn't seem to work)
-Create a setup in which three of the players are on the ground in close proximity.
-Use the 4th character to then jump on all the players in quick succession.
If done correctly, you should unlock the achievement.
Below is a video showing what it looks like in action: