Originally Posted by pvaudio
This corkscrew spin business is not easy to understand because you cannot picture it easily. Here is the best way to do so, IMO. Either imagine or put a tennis ball in front of you. There are 3 directions: x (left and right), y (forward and backwards) and z (upwards and down, aka into the air or through the table). Top spin is a pure positive y direction forwards spin, as in the ball will roll away from you. Backspin is the negative y direction, as in towards you. Side spin is a pure x direction spin, so either rolling to the left or the right. The corkscrew spins ABOUT the z-axis. It does not move in space, it simply sits where you puts it and spins. To picture that, imagine spinning the ball like a top. There's your z-axis or corkscrew spin.
Indeed, your analysis is correct. It's generating that extra component that causes the twist portion. It's how you generate it that causes all of the difficulty with this serve.
Very good explanation. Below is a visual aid that shows the 3 (reference) axes of rotation. This is a repost of mine from 2011:
Imagine, if you will, that this airplane is flying away from us -- we are viewing it from the back. We have 2 horizontal axes of reference. Let's call the Purple (Pitch) axis our X-axis. This axis would be akin to the axis of rotation for topspin and underspin. This axis is perpendicular to the direction of flight.
The Red (Roll) axis is our other horizontal axis. I will refer to this as the Z-axis. This would be the axis of rotation for spiral spin -- the axis is in the the Z direction, the same as the flight of the airplane or ball.
The Yellow (Yaw) axis is our vertical axis and is perpendicular to the direction of travel. This is the axis of rotation for the spin that we refer to as sidespin.