One of my topspin serves is done in the manner described above and to many players it looks a lot like a bottomspin serve. Ball toss a little over head, bat even with table top to 45 degrees, move bat forward to impact it just like underspin, but at impact, you are moving the bat backwards. The ball falling on a tilted blade will naturally have a bit of topspin, but when you pull the bat back at the same time, it generates much more topspin. The timing of this serve can be difficult to learn, but not impossible. Since many players serve underspin but cutting under the ball, following through forward and bringing the bat back, this serve has the same motion as a regular cut serve, but the impact timing is a shade different, kinda hard to catch. That is why it is effective on those who never see it. Plus, when you get good at controlling the impact, you can make this serve both low AND short for even more popups LULZ.
This serve is worth learning for the advantage it can give you (even if it wears off vs the better players) and for the learning of touch and timing that can translate into other shots.