This trick has no utility - it is only used to make a more intersting armageddon arena.

 

It can be done with any spell that causes lava (earthquake or volcano). Once you cast the spell, you must wait unitl the lava is spreading, and then you cast armageddon on that place, before the lava disappears. When the armageddon spell is used, the time will stop, freezing any lava on the map. If there is level where the spell was used, it will remain there, creating a lava arena. Note that the lava will not afect the units in the armageddon.

 

Watch the video so you can understand this better.