Yeah, it's just reversing the polarity of the second variable. Same as multiplying it by -1.
It's not the most elegant way to write this loop either, but it'll execute... For one thing, why you would declare those two variables outside the loop, just to turn around and redeclare them again...
Also, heh, just to bust your hump... This wouldn't be any different in As3 apart from the fact that it would warn you of the double variable declare.