@iEvo, it doesn't really work, at least not when it counts. Recently it only works with fall damage some of the time. getting killed by a mob and falling into lava doesn't make the tombstone appear like it used to, which is exactly why most people get it.
It's worked 100% of the time for me... I just jumped off a y=128 building, jumped into lava and got killed by a spider (all in a protected town) and they all spawned...