I'm not a ref, but I'm pretty sure it is intentionally throwing the ball to an area no eligible receiver is *while* being inside the tackles. Outside the tackles you still need to throw the ball at/to the line of scrimmage for it to be a legal pass. I'm sure I'm missing a crucial element here...
Of course, all standard rules apply here, such as needing to be behind the LOS, can't already have a forward pass on the play, etc.
