Contact point is the peak of the acceleration phase. The energy created through the kinetic chain is finally transferred into the distal wrist joint directing the energy of the leg drive, trunk rotation, shoulder internal rotation, forearm pronation and finally the wrist flexion into one special point of time and space.
At the contact point the ball velocity is produced by the wrist flexion, forearm pronation and internal shoulder rotation. But let’s start from the things that contribute the most to the efficient shoulder to wrist movement.
Hitting side
Elbow extension. At the impact point the elbow is not fully extended (around 160 of maximum180 degrees). This is important to understand as it prevents the elbow injury. But on the good side, if he elbow would be completely extended then it would slow down the velocity of the racquet.
Trunk. As elbow is not fully extended, the trunk has to be laterally flexed (to the left for the right handed players), at the angle that the racquet does make a contact with the ball above the hitting shoulder and inline with the front foot toes. Hitting shoulder and hips are above the non hitting, enabling the shoulder above shoulder and hip above hip rotation at the moment of he contact. 
Back leg. Back leg begins the back and away from the front leg movement to make sure that the hitting hip is aligned at the contact point with the non hitting one, but blocked from the further rotation forward in the follow through phase. 
Back leg ankle. Back leg ankle is in the plantar flexion position, pulling the leg backwards. Like in swimming, the more the ankle is flexible and able to bend past straight, the player can create more powerful back leg pulling motion.
Non hitting side
Non hitting elbow. As the forward arm is moving to the front to commence the forward shoulder angular rotation to the contact point, at the point of when the hitting arm is at the impact zone, the non hitting arm and elbow have a very important positioning task. They have to position so that non hitting and the hitting shoulder are aligned at the contact point, and to enable the further forward rotation of the hitting shoulder in the follow through stage.
How? 
The elbow and non hitting arm are pulled towards the abdominal area, pushing the lower abdominopelvic quadrants at the point of the impact, like in the swimming motion. In this way the both shoulders are leaned forward (which is prerequisite for the efficiency of the follow through motion) and it adds to the further forward and away pronation of the hitting arm. Elbow pushing against the lower abdominopelvic quadrant (left elbow against the left quadrant), contributes greatly to the support and stability of the non hitting hip and non hitting shoulder, preventing them from any sort of rotation during the contact point. This is a prerequisite of the efficient angular rotation in serve.
Front leg. Front leg provides stability so that the angular rotation of the hitting side is performed the most efficiently as possible. The stable front leg brings the stability to the front, non hitting hip, so the back hip can perform the rotation up and forward.
The front leg knee begins to bend forward (flexion) in this phase of the serve. In this way it brings the stability but it pushes the body forward as well, making sure that the front foot lands strongly inside of the court at the end of the follow through stage.
Front leg ankle. The ankle is in the plantar flexion position, which brings more to the stability of the front leg. The flexion is adding extra to the front leg pull to lend strongly inside of the court at the end of the follow through motion.
Next: Follow through and landing
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