What is the etymology of the word "pegging", referring to the sexual act?

A bit of a weird one, but we hear the phrase being thrown around a lot lately for whatever reason.

Closest reference I could find was in the Wikipedia entry for for Pegging (sexual practice) as follows:

"The neologism "pegging" was popularized when it became the winning entry in a contest in Dan Savage's "Savage Love" sex advice column, held after an observation was made that there was no common name or dictionary definition for the act in the English language."

I Google'd Dan Savage and Pegging, but found nothing. Any ideas? I can't make a connection between the word and the act - am I missing something very obvious here?