Android boot manager is new dualboot tool for android. There were much of them before, such as: EFIDroid, MultiROM, Dualboot patcher, etc. Usually they reflashed boot dynamicly for dualbooting. Some of them used one big hack like MultiROM's kexek-hardboot.
Android boot manager uses lk2nd flashed to boot partition for loading device tree, kernel and ramdisk. And android app with sh scripts and pre-compiled binary for easy rom managment.
For now project is in pre-alfa stage. Actively developed.
For now this is developed for Motorola Moto G5 (cedric). But a bit later i will create porting guide.