Many players understand the importance of using a semi-circular rotation of the racket to create deceptively different spins. For example, with a forehand pendulum serve, the racket goes through a rotation that starts by going down, and then down and sideways, sideways, sideways up, and then up. Depending where the contact is made you get different spins - backspin, sidespin-backspin, sidespin, sidespin-topspin, and topspin.
But you can get even more subtle than this. With the above motion, the axis of rotation at the start of the serve is the elbow, with the axis changing to the wrist just before contact. This means you get spin from both elbow and the wrist.
Now imagine snapping the wrist so that the tip is going down, but pulling up slightly with the elbow so that the bottom of the racket (near the handle) is moving up, with the axis of rotation in the middle of the blade, i.e. the blade spins about its center. If contact is made near the tip, you get backspin. But if you contact it nearer the handle, you get sidespin or topspin! It won’t be as much spin as with the standard forehand pendulum serve, but the mount of spin isn’t nearly as important as fooling the opponent. In this example, the opponent will see the big downward swing of the racket tip, and will likely instinctively read the serve as backspin - and so you contact it near the bottom, with some combination of sidespin and topspin, and watch the opponent pop the ball up or put it off the end!
You can use this same principle with any semi-circular serving motion, though you should first master the standard version. Note that quickness of the motion is more important than the amount of spin - the goal is to trick them into thinking it’s backspin, so exaggerate that motion. And then let loose with this new variation, and watch the looks of disbelief by opponents who were absolutely certain the racket was moving down at contact!