Direction of the stroke is related to the timing and angle of the impact point. The unwritten rule is that the ball will be following the direction to which strings of the racquet are directed to at the point when racquet intercepts the ball. Usually, in order to perform at their best, the players change timing of torso rotation to the ball while changing direction of the stroke
In order to keep lateral momentum as a source of power, the player needs to keep the inertia (lateral direction) without pulling the breaks before the stroke transferring the linear momentum into the forward momentum
For a player to move the body segments efficiently, there should be optimal combination of the stability and mobility through lowering the center of gravity created by loading the major muscle groups of the lower extremities.
Upper body force creation depends of the elastic capabilities of the muscles of the trunk. The reason the players create a larger upper trunk rotation beyond the hips is to pre stretch the trunk rotator muscles and create a powerful separation angle