I found this:
What's The Proper Way To Wave
There's a lot of different ways to wave and some riders attach meanings to various methods of waving.
The Low Wave - Extend the left arm downward at a forty-five degree angle (straight out is the signal for a left turn). I've seen this most often used by folks on cruisers, especially Harleys. There are variations of the low wave:
- A fist with index and middle fingers extended together - The low wave if you're on a two cylinder bike.
- All four fingers extended - The low wave if you're on a four cylinder bike.
- A fist with the index and middle fingers extended but spread apart - A peace sign meaning "peace on the road."
- Thumb and index finger making a circle other three fingers extended - The "OK" sign saying, "I'm okay, you're okay." Can also be a question asking if the other rider is okay.
- All five fingers extended, palm facing forward - the "low five."
The Two Finger Flip - Left hand remains on the grip, but the index and middle fingers come up. This is often used by crotch rocket types because the forward leaning position can make it a little awkward to take the left hand off the bar. Also used by bikes of all types in traffic when they don't quite feel safe taking a hand off the bar.
The Four Finger Wiggle - Left hand remains on the grip, but all four fingers come up and are wiggled side to side a little. Used in the same situations as the Two Finger Flip.
The Mini-Wave - The left hand comes a couple inches off the grip to flash a wave before going back to the grip.
The Full Wave - the left hand comes up to shoulder height or higher to wave.
The Vigorous Full Wave - The left hand comes up to shoulder height or higher and is waved back and forth vigorously.
The Biker Salute - Left fist held up high. I've only ever heard of this or read of it, but never seen it. The references that I have heard or read indicate that it's an old-time biker gang thing. I dunno how true that is or isn't.
The Peace Sign - The left hand makes a fist with the index and middle fingers extended and spread.
Personally I use the peace sign mostly. If I'm not the initiator and am returning someone else's wave, I'll tend to mimic the wave that they used.