Just checking on the laws of the game because I seemed to remember things about Offside had changed and this was the case.
"gaining an advantage by being in that position" means playing a ball that rebounds to him off a goalpost or the crossbar having been in an offside position or playing a ball that rebounds to him off an opponent having been in an offside position.
So if you're in an offside position already you can't be "played onside by a defender".
I don't think a lot of supporters or even pundits actually know all the laws.