Apparently it's a very important concept in compiler or driver design or implementation. I never heard it before. It's written in bright red as a warning on page 5 of https://llvm.org/devmtg/2020-09/slides/CIL_Common_MLIR_Abstraction.pdf and it was the key concept behind the patch that George Hotz didn't understand and thought was bad in https://youtu.be/Mr0rWJhv9jU?t=320 I tried to google it but I suck at googling because it kept wanting to tell me about the forbidden gymnastics trick instead https://en.wikipedia.org/wiki/Korbut_flip