As mentioned before, the split step is the decisive step, the first action that the player should apply to get ready in time for the shot.
Split step begins before the contact point of the opponent as soon as the opponent starts the follow through.
The reason for this timing of the split step is that if the player has this timely split step, he will be in time touching the ground when the opponent makes the contact. At this point the player already recognizes the direction of the ball coming towards him, the type of the stroke by the opponent (overspin, underspin or flat) and the pace of the shot (faster or slower), so while transferring the weight downwards and pressing the ground at that point, he decides timely how fast to push towards that direction, if he should be in the lower or higher stance or should he start the rotation faster or slower.
So at this point a player has decided how strong and fast he needs to position towards the ball.
The split step is a preparation step but the beginning point of the take back as well.
When the player recognizes the direction and the pace of the ball, he automatically turns the side of the body towards the direction, making space for the upper body part to turn and start the take back and lower body part to move towards the direction.
He turns the shoulders so the ball is observed in a sideways manner (upper body) and he completely turns the foot and hips towards the direction of the ball (lower body) and begins the movement while being well prepared to hit the ball.
In this way the player is completely ready for the incoming ball.
Upper body rotation
As soon as the split step is initiated the non dominant arm is taking racket to the take back. This makes sure that the both shoulders are turning instead only the hitting shoulder. The problem with the only one shoulder turn instead of both is the uncontrolled take back (too close to the body or going very much sideways behind the player), which leads to uncontrollable follow through and body rotation side to side instead in the forward direction. Lots of stress is applied to the muscles of the arm in this way and it can lead to numerous injuries (there will be one blog soon just explaining the concept of the non dominant arm in tennis)
Lower body rotation
As soon as the split step is initiated, the foot and the hip are rotated towards the direction of the incoming ball (unless there is a first cross step which will be explained in the later blogs). In this way, the dominant side of the body is fully loaded either for the transfer of the body weight through the ball or the movement at the out of reach wider balls.