I was trying to work something on it but I got stuck somewhere and I wasn't able to launch Kwin with both a debugger and the libinput backend. I was doing it in a very different way than @zzag suggested and I do believe that way is a better and more extensible to implement it. Right now I don't have the time to implement it, but maybe in a few weeks I might have.

I would assume the purpose of the effect is visualize an in progress trackpad gesture. So for example, when you're swiping with 4 fingers to change your desktop, the effect could already start moving your desktop by a bit, to indicate you're performing a gesture.