Look again. The reason they twist, as before, is that one side is longer than the other, and has to travel farther to get the same place. You are enlarging the inside diameter, so if the shaving is unbroken, it must be longer on one side. Shear face removes a 1mm wide shaving perpendicular to the surface. Skew takes the same 1mm difference over a shaving more than 1mm wide, making a gentler twist.