It's a term used to describe injury to the inside of the elbow. It happens so often in pre-teens and young teens from pitching the name has caught on. When the pitcher throws the ball, the elbow is in a position called valgus. This means the angle of the elbow puts the force on the inside edge of the elbow.

Sports experts think a breaking pitch requires more force and twist on the wrist compared with the standard straight pitch. The force through the wrist travels to the elbow causing this problem.