Originally Posted by chaika

Haha, thanks again for the input people. I'm going to take that it probably refers to a joining/falling into a paceline around here, which is why despite riding for years I probably hadn't noticed it. Good to know that there is a lot of regional variance and I like the idea of using it to alert people to one's presence too. Can't wait to get out there and observe some butt-slappin' tomorrow!