[_Jim]

Well he didn't know where the kill switch was so he yanked off the spark plug lead....

I've used the old 'screw driver to ground trick' to kill a magneto-driven engine before - *DO* make contact w/the chassis/cooling fins on the engine first!

[HiTech RedNeck]

Well it LOOKED safe enough. There was a big honkin' silicone rubber boot over the plug, one which was originally part of a set of hi-performance wires for cars. But at that zippy speed, it was probably developing 50,000 volts or so open circuit, enough to flash through the insulation or air gap.