pg 66 of my 5th ed 1942 is an almost exact duplicate
i was apprenticed to two older electricians, both passed on now for some time, who taught me the'backhanding' technique
it was known that muscles contract when electrified, so testing for voltage with the back hand would have one's arm contract back at ones self, as opposed to moving forward into something live.
as one of these gents imparted to me " If you slap yourself, it's live!"
~S~
