Charley, the bowspring is a mooring line between the ship and the jetty.The onboard end is at the bow (usually) and the jetty end is towards the midships and when heaved against it pulls the ships head towards the jetty, if reversed it has the opposite effect (as in this case) and the ship is "backing the jib" to get her head off the jetty then it helps a lot, Hence the name It "Springs the bow off the mooring". In conversation with Mike O'Connor last year we were discussing this and he told me that the land breeze at Gibraltar blows onto the Jetty (still the same since Trafalgar)and tends to keep vessels pinned against the quay, and you HAVE to "back the jib" to get off if using sail alone, in fact the song idea came from Mikes own experiences of doing just this while sailing from Gibraltar! which started him thinking how they managed in the days of "Sail only" powered ships.