Single point incremental forming (SPIF) is a die-less sheet metal forming process in which blank sheets are locally deformed by using a generic forming tool moving along a pre-defined trajectory. Compared to conventional sheet forming operations, this technique achieves higher formability, greater process flexibility and reduced forming force. This is not only due to the characteristic of localized deformation, but also is because of higher through-the-thickness shear. In this work, in order to further investigate the influences of through-the-thickness on formability, a recently developed fracture model is utilized to simulate the forming of a 70° cone by deep drawing and SPIF. As a result, the predicted fracture depth of the cone formed by deep drawing is lesser than that of the same cone obtained in SPIF. It demonstrates that through-the-thickness shear in SPIF is much higher than the one in deep drawing process. Furthermore, a series of experiments in SPIF with tool rotation are performed to physically validate that through-the-thickness shear is a positive factor to formability when it is within a certain range.