Linear momentum in the serve motion is achieved when the whole body has reached the height from where the player can produce the perfect upper body turn (angular momentum) and create the most efficient follow through movement. For this to happen, the legs should create such a powerful drive that can transfer the energy through the hips, shoulders and eventually arm and wrist (kinetic chain)
Leg drive begins with the creating the forces pushing off the ground. As the Newton law states and in the general is well known, if you want to jump first you need to bend the knees and push off the ground.
Pushing off the ground would happen only if there would be a weight transfer to the toes, having the heels up in the air.
Automatically the knees are bending forward creating enough body weight pressure to the ground. So when you are told to bend your knees, that in fact means pass the weight to the toes.
In most common cases, the front leg is the stability leg and the back leg is the power leg. Power leg is the most important for the linear movement (jump) and front, stability leg is the most important for the angular movement (upper body rotation) as it provides stable non dominant side around which the dominant side is rotating.
The back leg is pushing off the ground, making enough energy to straighten up the hitting side while jumping up and having the back hip above the front hip which is very important for the efficiency of the upper body rotation and balance of the body while serving. With the back hip above the front hip, automatically the hitting side of the body is above the non hitting side (shoulders as well). 
In this way the angular movement from the hips and shoulders (rotation) is forwards, balanced and controllable in contrast with the rotation sideways when the hips and shoulders are aligned.
With the balanced forward angular momentum the follow through is controlled as the right shoulder is finishing in front of the body, arms sideways and back leg backwards.
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