...important for most riders IMO (tapping on the head/helmet)...
I'm sure you guys have seen motorcyclists wave at each other on the road but I'm not sure if you've seen or noticed the tap on the head/helmet hand gesture?
When you are behind a motorcyclist and he taps on the side or on top of his head/helmet when another motorcyclist approaches him coming from the opposite direction or vice-versa, it's not because there's a bee stuck in his helmet or he has water trapped in his ear or he has a headache...
It's mainly because he passed by a cop or a speed trap so he's informing the other motorcyclist to be alert: