Tapping the helmet is ussually a way to get your attention. sort of like "request to send" followed by "be on the look out" (pointing ahead) ussually this means cops.
Other common signals
Palm flat, hand low, moving up and down. slow down
Tapping gas tank - need fuel
Leg pointed out to one side- hazzard on that side of the road (gravel or dead animal)
Both legs pointed out - hazzard over the entire lane
Finger pointed to either side - Going to change lanes, this is the spot I'm going for. Most after market turn signals arent nearly bright enough so this helps
Hand in a fist pointed up at an elbow- going to stop, ussually by leader to the followerd.
Hand wrapped around an imaginary can, moving toward helmet- want to stop for a drink?
Thats all the ones I can think of at the moment