Everyone knows this exists in the X-Box, and PS2 versions, and everyone knows it is a difficult Fatality to perform, but I have figured out a few pointers that may help some who may not have been able to perform it, perform it almost anytime they wish. Simply of course pick Johnny Cage, and when prompted at the end of the match perform his Fatality move. Get in close, and press Forwards, Forwards, Forwards, and High Punch. Cage will crouch down and knock off his opponent's head. To knock off another head, you must do the Fatality motion again as soon as he knocks off the head. Another trick is when you hear the first scream from the opponent is when you want to start the motion over again. The final trick here is you must hit the last Forwards motion at the same time as the High Punch button. So the next motion will be Forwards, Forwards, (Forwards + High Punch). Almost every time you will score an extra head. Sometimes you may miss the second time, and uppercut plain air as opponent is falling down. There you have it. Press 3 Forwards, High Punch to do the Fatality. When the opponent starts to scream in agony, or as the head is comming off press 2 Forwards, (Forwards + High Punch).
Hope that may help in accomplishing this Fatality whenever you wish.