After achieving the proper launch position, the next move is to extend the arm upward by opening the elbow. There is absolutely NO RACKET MOVEMENT here. We call this "Saving the Hit", because you aren't using your hand and forearm yet. That comes in the next phase. So you need to hold back the desire to prontate hand and forearm. Hold back and "save the hit" by simply extending your arm through a straightening of the elbow.

This arm extension upward with no movement in the racket ahead allows you to get under and to the left of the ball. This is what lets you pronate almost directly from left to right. If you comare the angle of entry for a kick serve to a flat serve, here is what you see. On the right is a flat serve because the edge of the racket is facing the ball. This lets you pronate flat into the ball. On the left is the kicker where the strings of the racket are almost facing the net directly.