When something rotates, there will be a line where there is no movement. That is the axis of rotation. So when a player hits a forehand, the axis of rotation of the racquet will be between the middle and ring fingers, and perpendicular to the ground. Waggle a racquet in your hand and feel the alternating pressure on these two fingers, which predominate in the grip.
The shoulder is another axis of rotation, and this axis is used in computing Shoulder Pull.
On impact, there will be a resultant Torque about the hand axis of rotation, even if the impact is right on the sweet spot. Moment is a twist downward due to the racquet’s weight, and the axis of rotation for Moment is perpendicular to that for Torque, and parallel to the ground through the hand.
Yet another axis of rotation is along the centerline of the racquet (through the handle and the head), and it is about this axis that racquets will twist when the impact is off the centerline or even, due to the cross product of Moment and Torque, for impacts directly on the centerline. The twisting force about the centerline axis in the handle is known as Longitudinal Torque, or Torsion.