Originally Posted by JohnBrooking
Huh. I never would have thought of that either. What's the connection between the gesture and the meaning?
The slap is just to make noise and catch attention. A couple slaps on the left cheek means a group is passing on the left.
I'm not 100% certain of all this but I've seen pacelines of groips training do this as they pass other riders. It's also limited to just pockets with the country and not universal.