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 kexec-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-alpha stage. Actively developed.
For now this is developed for Motorola Moto G5 (cedric), but a bit later I will create porting guide.