Legal Pass?? Call me dumb

[BoBo]

I just read on another sight the following play.

QB A1 is scrambling and crosses the neutral zone then sees A2 open downfield. He then retreats behind the neutral zone again and throws the pass downfield. Which in the example B2 interferes. What is the result of the play?

Call me dumb but I always thought that once you crossed the neutral zone that legal forward pass restrictions end. I did not know you could go back behind and throw legally!!??

Hope you understand the play and please comment. I am frantically looking in rule books now.

Help me out on this one!