I have long wondered why "Stop" is not the normal gesture.
THIS.They were originally designed for cars before mechanical turn signals existed.
It is now legal, at least in Indiana, to signal a right bicycle turn with your right hand.
We needed a law for that?
-- or --
What's the signal for, "I'm going to ride my bike on the sidewalk straight at pedestrians"?
THIS.
In a car it is hard to point over your head and be seen.
They're also for HD riders too cool to use the actual equipped turn signals. Remember, if you're a "cool" HD rider, left is not parallel with the road, but at a 45 degree angle down so it looks like you're waving.