I've heard about this. Never tried it though. If i'm right, you take some of your saliva under and put it on a slide and then under a tiny microscope you look for certain patterns or something? I remember thinking it was a bit technical for me and i would obsess that i wasn't reading the pattern right or something. Then i scrolled down the page and it said something like not all women will develop the right pattern or something even if they are ovulating. So i stopped reading. Sorry not helpful lol.
But in a nutshell, i'm sure they are just as tricky and have their own problems like opks. It just comes down to preference on which best suits you and your testing. Some might be more sensitive to certain tests. Do you purchase it once and use it over and over again or do you only get enough supplies for a month or however it works?