The "pointing down" is actually the rider saying "Thanks" for the extra room. Take a closer look next time and you'll see the "pointing down" is actually done with both the index and middle finger, sort of like a downward pointing "Victory V". It's known as the American biker wave and is the same signal that's given by one rider to another rider to acknowledge the other rider and say "Hi" while riding.
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It's not done just by Harley riders.
