the way it's suppose to work is to hold the pwr key until you hear the windows 'chime' sound, that means you've disconnected from EDL.
Right after you hear the 'chime', release pwr key and hold vol minus key, that should then take you directly to fastboot.
Keep in mind, you have to flash the engineering abl to both abl a and abl b. This is because you don't know which 'slot' (a or b) is currently 'active'. So, if you only flash it to one of them, it may be the other one that is actually booting, thus it won't work.
Also, when backing up both a and b, be sure to name them appropriately, to return them to where they were, as they may be different versions of android.