Spin Loft is the difference between the Dynamic Loft and the Angle of Attack.
Technically, it is the difference between the 3D Club Face and the 3D Club Path, but, for the sake of simplicity, we will think about it in two dimensions.
The greater the Spin Loft, the more the ball will spin, up to 45*. Beyond 45*, the ball actually spins less due to a loss of friction.
