Nearly there Dave,
So it's off with the plug cap, out with the pencil and draw a thin carbon line from top to bottom of the ceramic insulator.
As we all know HT current will always take the easiest route so instead of tracking down the centre electrode and hurling itself across the correctly set gap it runs straight to earth thereby bypassing a spark situation.
Then stand back and watch the victim remove and strip the carb. etc etc.
Used to have a lot of fun doing stunts like this particularly in the old days especially when you had a BC ( bus conductor ) lookalike.
Sometimes hours of fun to be had again at someone else's expense.
Works on multi cyl. outboards too.
And old V 8's.
Remembering The Good Old days, when Chiefs stood watches and all Torque settings were F.T.