Well, if they are dragging a plastic bag by the pipes......
you get their attention.
Make an "O" with one hand...........
Then lift yourself up and hold same hand under butt.

Here's one guess,
Get the attention of fellow rider.
Point to your eye, your Knee, then your tank.
(I Kneed Gas)
(If you point to mouth last = I kneed food)

get attention fellow rider,
Point to him then yourself, then knee, show peace sign, then point one finger up and do a circle.
(We kneed 2 turn around.)

Another .
Get attention,
Then hold hand in a fist then show 5 finger spread, do it over and over.
(Your blinker is still on.)

Tapping your helmet for LEO? I'd think you were adjusting something.
I put my hand out flat by my knee and wave it downward, Like slow down slow down slow down.

When leading,
Put hand on top of helmet and hold it there a few seconds and flash brake light several times.
Like (Oh crap something is going on here! And point to it, road hazard, traffic Jam, so on.)

On Super slab,
Turn on signal and leave it on although no lane change is going on. And flashing brakes.
(Deer or some jerk about ready to pull out in front of us so watch em. Left signal from the left right signal from the right. or quick left right left right means both sides)

I have all kinds, but no one else seems to know what I am talking about. So that's normal!