It's a new mode in split screen. Currently you can have a 50:50 split, where each app takes up half of the screen, or a 70:30 split where one app takes up more than the other. The 90:10 split is a new setting where you can have one app take up almost all of the screen with the other just taking up a sliver, and then you can simply tap on the smaller one to swap them - makes for quick multitasking between two apps. It looks to be similar to OnePlus' Open Canvas but not quite as powerful. Looks like it will now be coming in QPR2. I believe it's already available on some Samsung phones.