The mind boggles! Presumably Brophy meant both the phrase and the gesture were a euphemism for the sex act, so he implies there was an innocent explanation and a sexual one. All the ones I can think of are too explicit. What could have been 'the sex act performed illicitly'? Gay men perhaps?
I have a vague recollection of holding the left hand, tips of forefinger and thumb touching to form a ring, and then putting in and out of the hole the forefinger of the right hand, to denote the sex act, and to an innocent this might not be construed in the same way, but this would not explain the phrase. Intriguing.