Back in the year 2000 when I started riding a motorcycle, somehow I wasn't prepared for the pleasant surprise of the "motorcycle wave" phenomenon. The first few times it happened, I really thought for a moment I must know some of those waving people. Many "waves" later and some of my new rider friends (yes YOU Rex) comment on this phenomenon and I try to define it but I can't do as thorough a job as this man did on his blog below, very funny.
Secret Motorcycle Hand Greetings ~ Revealed
and then I discovered this lovely "research" on the BMW MOA website about hand waves as well.