I'd guess the clever bit must be in how much power you can feed down that tiny wire without over egging the pudding with Volts and lengthening the spark or over ramping the Amps and melting it. Lots of sparks per second seems to be better than a few fat ones, so you need low inductance and presumably power it from both ends of the wire which sounds tricky 'cause one end is under water and has to reach around the part being cut.