The wrist "snap", or rather the flexion of the wrist during the service motion, is a very debated and often misinterpreted topic in tennis. Biomechanical research shows that at the moment of impact there must not be many moving parts, the whole must be essential and "solid", therefore there is nothing that can be compared to a rapid and forced movement of the wrist to influence speed or trajectory of the serve!
When we are in the phase of maximum external rotation of the right shoulder (i.e. when the racquet is behind our right shoulder, totally vertical and with the head pointed towards the ground, also called "racquet drop" phase) we are in the "point of no return" before the actual acceleration of the racquet towards the impact. Subsequently, an internal rotation of the shoulder takes place, which together with the pronation of the forearm (the movement that rotates the palm of the hand from "facing the ear" to "facing the right side") the racquet to have the string-bed turned to the other side on impact.
01. Maximum external rotation of the right shoulder (or racquet drop phase);
02. Internal rotation of the shoulder, pronation of the forearm also begins;
03. Shortly before the impact, the pronation of the right forearm begins (to avoid "picking" the ball with the edge of the frame), the internal rotation of the shoulder continues;
04. Closure of the pronation of the right forearm, deceleration phase with the end of the internal rotation of the right shoulder.
Since the acceleration phase is not controllable, as it takes place in a few milliseconds, it is what we do before that guarantees a good success rate. Of course, I can internalize the correct movement by doing "shadow swings", or shadow movements, without the ball that are performed very slowly, to "feel" the correct movement.
An exercise that can help us "put the pieces together" of what we have seen so far consists in simulating the throwing of the weight with a tennis ball.
We start in the service position, left front foot parallel to the base line, and we immediately go to the position of maximum loading, with the weight at 75% above the rear leg (right), the elbow high and away from the body at about 90°, the right hip rotated clockwise with respect to the base line, the left arm stretched out and totally vertical, the tennis ball in the right hand, holding it without squeezing it with the fingers, but simply resting it on the palm. Here, too, during the launch, we make sure that the shoulders are one above the other, not side by side at the same height.
01. "Throwing the weight". We start in the service position, left foot parallel to the base line, and we immediately go to the position of maximum loading, with the weight at 75% above the rear leg (right), the elbow high and away from the body at about 90°, the right hip rotated clockwise with respect to the base line, the left arm extended and totally vertical, the tennis ball in the right hand, holding it without squeezing it with the fingers, but simply resting on the palm.
02. During the jump, make sure that the shoulders are one above the other, not side by side at the same height. The "shoulder over shoulder" position should be emphasized in any progression and that should lead us to correct the service movement.
03. When we feel comfortable with the loading and launching movement, we can insert the "jump" upwards and forwards, also emphasizing the arabesque, or the upwards "kick" of the right foot. This guarantees that we have performed the jump with adequate explosiveness, and allows us to work on the exit in balance from the movement of the service.
While throwing we do not "jump" (we will stop at the step illustrated in image 02 above), let's just perform the movement by releasing the twist of the lower body and taking care of the moment that corresponds to the impact, stopping at the moment when the ball leaves the palm of your right hand.
The line joining the shoulders must be perpendicular to the ground, not parallel to it.
The goal is to throw the ball as high as possible, not as far.
Once we feel comfortable with this movement, we can also perform the exit jump (image 03, above).