The picture above was taken by Alexei Watson who states that the new stock recovery in Android L has a few new features. You can now Reboot to Bootloader, and Power down.

First off, the bootloader isn't part of Android, it's what lets the device boot into Android. Device manufacturers generally want you to stick to the version of Android they preloaded on the device.

The unlock button will reboot your device, unlock the bootloader, wipe all your data, and then return you to the Android 4.1 setup screen.

If you want to unlock your phone’s bootloader, then you’re in great luck–we have a step-by-step guide on how to unlock the Galaxy Nexus’ bootloader.

Rooting Process: To root the OnePlus 3T and OnePlus 3, users have to unlock the bootloader, and then have to proceed for the installation of custom recovery and SuperSu/Magisk.