First of all - yes: topspin volleys are quite unusual, difficult to play, and not necessarily something your average pro teaches you. They tend to be risky too, but their wow-factor and your personal feeling of satisfaction after a successful attempt make more than up for it :) And if you make them part of your game - like the Williams sisters have - they can be very effective to finish off well-prepared points.
When I used to teach, I liked to mix in topspin volley drills for my advanced students, mostly for fun! Sometimes I would even park my car next to the court, open the trunk, and play some "inspiring" music to go along with it... The good old days. Anyways, so here's what I think is the 2nd most difficult shot in tennis: the one-handed backhand topspin volley:
Right-handed:
Left-handed:
What helps to play this shot well? Well, first of all you need a high ball coming your way that gives you enough time to prepare. You then go for a normal topspin backhand, however you want to make contact with the ball above net-level. Shoulder level works best for me. You obviously want to hit a powerful shot, but the ball needs to come down, so you need spin. Consequently, make sure that the racquet head is perpendicular to the ground, and your arm fully extended at the point of contact. After the contact, move the racquet head upwards. In the video above, I pull the racket slightly sideways after impact, which might just be my personal style. The most important thing is that the racquet head brushes upwards against the ball after making contact.
If you have a 2-handed backhand, you might have seen backhand topspin volleys from the Williams sisters and other pros. The motion around the point of contact is quite similar, however you don't make contact as far in front of the body, and end up with the racket head in your "backpack" as you would with the corresponding groundstroke.
Try at your risk - if you haven't tried this shot before, better start in practice (and don't forget to have fun with it)!