Am curious. Whats the sign? 
Rule of the Road?
There seems to be an "adopted" protocol of biker to biker recognition between/among riders that has to do with a slight, horizontal pass / wave of the left hand upon cross-traffic passing one another. You know, the slight wave of the left hand at hip height across a horizontal plane....an acknowledgement from one rider to another....where does this come from? is it something that is expected, a simple sign of respect, courtesy, etc? I do it occasionally and sometimes I don't, a lot depending on what aspect of motion I am undertaking at the time, ie. slowing down at an intersection, using my clutch, etc.....then again, sometimes other riders give the signal while others don't - so what gives?
I assume these are serious questions? There are no hard and fast rules that I have ever seen. Each rider develops his own style .Many Harley riders use the old hippie peace sign by holding out two fingers in a V formation. Many show it at waist level or below because they are afraid if they raised their hand up it would look like a Goldwing rider or something.

I believe the wave thing started in the very early days of riding because it was rare to see other riders and it was acknowledging the brotherhood of riders.
Some riders only wave at like brand of bikes and refuse to wave at sport bikes etc. Other wave at all riders remembering that we all share in the brotherhood of riding.
Some accidents have been caused by over eager riders trying to wave while cornering or doing some maneuver that should have their full attention so you are smart for not waving when you feel unsafe doing so.
One grey area for me is when you come upon a large group of riders coming toward you in the opposite direction, I wave to as many as possible but if I get tired I quit waving and hope i don't offend anyone in the group by not waving to them. I don't loose sleep over it though. For those times we need a old time curb feeler on the handlebar with a glove on it so it waves for us.
