The exact technique to enter / boot into Fastboot on an Android phone varies with the type of device. On HTC (for example), you'll first need to enter Hboot . For Intel based Dual OS Tablets, you'll get into DNX (Fastboot) mode . On very few devices, Volume down + Power button combos will do the trick. You can also get an option to reboot into Bootloader from recovery mode on some Android phones. A generic method (which cuts across most devices) is using ADB.
Steps to boot into Fastboot mode using ADB
Follow the steps below to enter fastboot using ADB