I've read and re-implemented a lot of stuff by Inigo Quilez. Much of it is ingenious but eventually straightforward once you get the hang of it. I still find this particular snippet magical:

https://www.shadertoy.com/view/MdB3Dw

Well, the code itself is simple GLSL. The math is (and I have not verified this) not that complex either, as I believe it's all about integrating the raytracing equation for a moving sphere over the time variable. I guess what I find most impressive is just the fact that it works and that someone even thought of doing that. I also think this is currently the only example of this method being used anywhere, and it might mostly stay that way because the method might not be super generalisable.