I think that aiming your left hand down would signal "I'm stopping", which could be problematic if the motorist decides to go around you and turn right. After all, you indicated a stop. Perhaps signal a left turn and go straight? It might tick them off but I'll bet the WTF is he doing will make them hesitate to hook you.

For what it's worth, some cyclists just point where they're going and they say it works. The only downside (this is second hand) is when you point left to signal and the motorist thinks you're telling him to go around you on the left. It might work pointing forwards though, at just enough angle for the car behind to see it. (?)