Late Forearm Turnover
The term was originally coined by Dr. Mike Marshall as "late pitching forearm turnover."
As the name suggests, this is an arm action in which the forearm moves into the laid back (throwing) position after the body has begun to accelerate the arm.
When the forearm turns over late in the 'traditional' delivery, it tends to increase the inertia that causes the forearm to lay back. This inertia is the primary contributor to ulnar collateral ligament injuries when it causes reverse forearm bounce.
This action almost always (but not exclusively) occurs after these inverted arm positions: Inverted W, Inverted V, and Inverted L. These positions usually involve both very late forearm turnover and powerful external rotation, two actions that lead to increased valgus force.