One of the most basic and yet misunderstood techniques at the intermediate level is the semi-circular motion used in serves for variation and deception. For beginners and beginning-intermediate players, the key is getting lots of spin, such as backspin, backspin-sidespin, sidespin, sidespin-topspin, and topspin.
While learning to do these spins there's little attempt to disguise the spin - the racket moves in one direction, the direction of the spin. But once you can do that it's time to learn to use semi-circular spin to disguise the spin. For example, for a forehand pendulum serve (with racket tip pointing down), you would start with the racket moving down, then down and sideways, then sideways, then sideways and up, and then up, like a pendulum. If I were to show you this motion and ask what spin it is, it's a trick question as you can't answer without knowing where in the serving motion contact was made.
Here's a video (7:24) showing this type of semi-circular motion serve by top players in slow motion. Note that it's almost impossible to really study a top player's serve except in slow motion - at full speed everything happens too fast to see what they are really doing. In the videos one thing to look for is the amount of motion the racket does in directions different from the direction it goes at contact. Often the actual contact happens so quickly that even in slow motion it's difficult to pick up as the racket is rapidly changing direction at that point to confuse the opponent. The direction the racket is moving at contact is usually only for a split second, while the rest of the service motion exaggerates the racket moving in other directions. (Also note how after contact the racket often goes off in a different direction, sometimes away from the semi-circular motion, as an additional attempt to disguise the spin and confuse the opponent.)
At the higher levels, at full speed, the serves often don't look like semi-circular motions because the motion is done so quickly with a much shorter motion. But in slow motion you can see the racket go through this circular motion. It's not always a complete semicircle as parts of the motion are shortened for disguise.
This one of the three important ways of disguising the spin on your serve. The other two are sheer amount of spin (making it hard to precisely read it), and spin/no-spin combinations (see my article Those Dizzying No-Spin Serves).