I think all the posters above have made the point about the impact that your stance (open, closed) makes on the path of the racket as it cuts under the ball. If you cut under and then follow through in the direction the ball came, the ball goes back in the same direction and you put pure underspin. But the more you cut under and across it (because you are more open, or because you want to change direction - say, the ball come to your backhand but close to the middle of the baseline, and you want to slice it down the line to the far corner), the more sidespin you add also.
Some posters have talked about putting sidespin on the outside. The shot you can do this is when you take a high backhand early, and you slice it down and across to bounce short (rather than at the back) in the opposite service box. The way you do this is by slicing around the outside of the ball (which puts sidespin), and because it's a high ball you're hitting you're actually slicing slightly downwards. The result is that when the ball bounces it tends to check and it also spins sideways but parallel to the net, so ends up like a bit of a dropshot.
I find this type of return very effective when playing mixed doubles (where I usually take backhand), on the lady's second serve. It often is a bit of a floater (just to get it in), so I first put pressure on the server by stepping right in to take it early with an inside-out forehand, as she is preparing to serve. Then, if she manages to plop her second serve to my backhand and I don't want to give her extra time by running around it, I simply step further in and cut it down, short and across, aiming for the sideline of the singles court, about half way up the service box (ideally, shorter, so more of an angled drop). I know that she will never play S&V on that serve, as I'm threatening the hard return, so her momentum will be to serve and hang back, so she'll be late in starting to run forwards. If she gets it, it's still a tricky backhand she has to pick low and upwards, and the most she'll be able to do is to pop it up in front of her, for my partner to kill on the volley. If she manages to steer it across, as I'm coming up the ball will play into my strong, high forehand volley; or if she does manage to lob my partner, the lob will still be slow-paced enough for my to switch and go back to cover. Really, the only effective answer is a low dink diagonally across to the opposite sideline on my side of the net - not easy to do when you're stretching on your backhand...
