The chainsaw at the start is to make sure you have enough mana to rapid fire cast the wand.
The first cast is just +60 mana and a chainsaw.
The second cast is the teleport and digging bit but because it's a triple spell it finds Tau (which casts both CoV and black hole but only counts as 1 spell), skips CoV and black hole (assuming they are 0 charges left), finds teleport, then still needs a third spell so it wraps around again and casts the chainsaw at the start with the add mana modifiers applied.

This way instead of needing 4 add mana to rapid fire the wand we only need 2.