Teaching the forehand pendulum sidespin-topspin serve
Teaching the forehand pendulum serve is easy. Most relatively new players learn to do it with backspin and sidespin-backspin without too much trouble. But serving it with sidespin-topspin? This might be the single most difficult thing to teach. It's like teaching someone to whistle - at first they try and try, and nothing seems to work, and they get frustrated. And then, suddenly, it just happens, and then they get it, and from there on it's no problem. The same is true of this serve; players often struggle and struggle with it, which is frustrating to the player and the coach. And then, it just suddenly happens. I'm not sure why this particular skill is so much trickier to teach than other skills. You'd think that teaching a loop would be harder, but I've found that's much easier in practice.
The basic idea of the serve is that the racket goes through a pendulum motion. To get backspin or sidespin-backspin, you contact the ball on the downswing. To get pure sidespin, you contact the ball between the downswing and upswing. To get topspin or sidespin-topspin, you contact the ball on the upswing. To maximize spin, bring the wrist back and smoothly snap it into the serve, like a whip. It's helpful to imagine your arm (just above the wrist) hitting a pole just before contact, so that the wrist and racket whip about like the tip of a whip, or a tetherball spinning about a pole as it runs out of rope.