If you've been modding your Android devices for any amount of time, you're probably familiar with Chainfire, developer of the extremely popular SuperSU root permissions manager and various other mods and apps. His latest work attempts to solve an annoyance that has bothered Android power users for years, but has become particularly annoying since the introduction of Android 5.0. Chainfire's new experimental method for rooting Android phones, tablets, and other gadgets does so without modifying any of the files on the /system portion of the device's storage, instead using a modified boot image.
That's a big deal, because after Android 5.0 any modifications to files in /system (even without a permanent root solution) cause over-the-air and sideloaded updates to fail, forcing rooted users to stay on older versions of Android or re-flash the stock build, then root again. With this new method for Android 6.0, a user could root his or her phone, use it for however long they liked, then flash the original boot partition via ADB or other means. Then they could apply an over-the-air update without having to either flash an unmodified system image or simply install a full factory image. It's not "simple" in the one-click sense, but it would save a ton of time and effort that's currently devoted to setting up a newly-wiped device. There's also at least one report that this method gets around SafetyNet root detection in apps like Android Pay, though the developer says that's unlikely to last long.
Chainfire's modified boot method still has some kinks to work out. A lot of apps with hard-coded paths to Super User don't work, a factory reset of the phone wipes both the root status and the phone's PIN, each phone needs its own individually-modded boot file, and Chainfire describes the current development status as "bugs, bugs everywhere." There's also no way to apply this to a currently-rooted phone without flashing a stock build again. Even so, it's an exciting development for root users - hopefully some of the problems will be addressed before the next big Nexus OTA.
You can read the instructions for the "systemless" root at XDA, where Chainfire has posted the necessary files for the Android 6.0 builds on the Nexus Player (fugu), Nexus 5 (hammerhead), Nexus 5X (bullhead), Nexus 6 (shamu), Nexus 6P (angler), Nexus 7 2013 (razor and razorg for Wi-Fi and LTE versions), and the Nexus 9 (volantis and volantisg).