Hi-Lo (or Higher-Lower) threads are twin screw threads, but exactly where one of the threads is lessen than another. The idea is to deliver a screw with the bite and generate speed of the twin thread but that's not as likely to separate the material which is currently being screwed. https://typesofwoodscrew37936.blogtov.com/9829415/fascination-about-types-of-screw-head-punch