[Lynn Eades]

In the old days, getting the spark to jump a 1/4" inch gap was your lucky day, and shorting out the plug was no big deal. Any "man" could handle it!
Things have improved a bit and now the spark will jump about 3/4" (for the metric thinking, that may as well be a meter)
The idea of shorting these ignitions out with a screw driver sends a wee shiver (induced current?) up the spine of any life loving mechanic, at the preliminary thought / idea of shorting a plug out, and makes him /her (plus 20 other pronouns) wonder how much truth there is in those warning stickers about the dangers of meddling with these voodoo boxes.
Btw, for the newbies; stick the sharp end of the screwdriver hard to the head (earth point) BEFORE introducing the spark plug lead to the shank. This generates more job satisfaction, for you. If you do it the other way around, it generates job satisfaction for other people.