Not interfering. And it has to be that way otherwise you could never have a run to the byline and cutback because the winger would always be offside.
This is different because it goes so close (through/under his legs?) and is clearly in the line of the ball and goalkeepers eye.
You can't be offside if you are behind the ball when it's played to you. As Cloughie said, if a player isn't interfering with play, what's he doing on the pitch. Interfering/not interfering is far more subjective thing, though I guess there's more excuse for the bloke who's on the floor.