Android Boot Manager is a new dualboot tool for Android. There were many of them before, such as: EFIDroid, MultiROM, Dualboot patcher, etc. Usually they reflashed boot dynamically for dualbooting. Some of them used one big hack like MultiROM's kexec-hardboot.
Android Boot Manager uses Little Kernel flashed to boot partition (or sometimes as bootloader replacement, you will be warned if so) loading the device tree, kernel and ramdisk, and an android app with sh scripts and pre-compiled binary for easy rom management.
For now project is in alpha stage. Actively developed.
For now this is developed for Motorola Moto G5 (cedric) and Volla Phone (yggdrasil), but a bit later I will create a porting guide.