As others have said that is wrong, but the situation almost never arises.
How is it wrong?
Say there is 2 strikers and the opposition goalkeeper no defenders are involved.
The goalie comes out and the player slides it to the other team mate. If player B is behind player A and the pass is "backwards" then he is onside right?
Yes but if B is in front of A (would have to be a shit pass) and the ball goes backwards, B goes back and gets it, then it's offside. Hence it being a hypothetical situation that's very unlikely to happen.