Originally Posted by Homebrew01

The elbow flick is to signal that you are pulling off and the guy behind should pull now. It's not usually needed in a cooperative, team or group setting. More common in races when the guy behind is on a different team and might not want to pull, or the group has the whole road and it's not always clear if you are taking a different line, or pulling off. I find it annoying when people do it on group rides.