which s/h in their right mind would give up 5 metres? For me it's a lateral line across the pitch and is just there to ensure the s/h on the side losing the strike cannot close down on the other side.
I see it quite frequently !
I assume the reasoning is that if you retire to the back foot you can now defend both blind and open side
Sometimes I will see the SH moving laterally on his offside line, ie away from the scrum to cover the opposing 10 (not so common )