In the pacelines I ride, it's a signal to indicate move left because there is an obstruction on the right (like a parked car).

I did use it once in a pace-line and it confused the driver next to us enough that she pulled up to us at a stop many miles later to inquire what it meant (she was the nicest person, she was just curious about the signal and by the end of our conversation I had pretty much almost convinced her to join us at our next group ride).