The direction of the ball’s spin after release as it travels to the plate. Spin Direction is defined as the direction of motion of the leading edge of the ball (the side of the ball facing the batter as it moves to the plate).
How DK Shows It:
In the PitchTracker Baseball app, Spin Direction is displayed graphically as a spinning arrow around a baseball. The arrow spins in the same way that the pitch would appear to spin from the pitcher’s perspective. In other words, we are showing spin just as you would see it after you threw the ball.
This means that the “leading edge” of the ball is hidden from the pitcher’s view, so the “spin direction” is the direction of motion of the hidden side of the ball. In order to quantify the spin direction, we use a clock face. The spin direction points to a number on the clock (that is, the clock time nearest to the spinning arrow as it comes into view from the leading edge of the ball is the spin direction for that pitch).
Ball spin is what causes the ball to break due to aerodynamic forces acting on it during flight. The direction of break is the same as the spin direction. The amount of break is dependent on the spin rate, so knowing both of these values tells you about both the direction and amount of break that you can expect from pitches.
Real Life Example:
A fastball has backspin, meaning that as the ball moves away from you, the leading edge (side toward the catcher) is moving upward as it spins. We define the direction using a clock face. 12:00 is the top of the clock face, so an ideal fastball, would be measured as a 12:00 spin direction.
A curveball, on the other hand, has top spin, meaning that the leading edge of the ball moves downward, toward 6:00 on the clock. So an ideal curveball spin direction is measured as 6:00.
Your actual curveball and (and other top spin pitches) probably have spin directions that are slightly different from 12:00 or 6:00, which is fine. That makes you unique as a pitcher. Knowing the spin direction of each of your pitches helps you understand how they actually break.