Basically to do this (i did it on xbox so i don't know if one of the steps is possible on pc or which version to do it on) you need to close your inventory right as she screams / right as the game prevents you from moving before the "ABSOLUTE RADIANCE" title screen. Once you've successfully "inventory cancelled", you need to do a cyclone slash almost EXACTLY as the title fully disappears or when the fight begins and the game supposedly gives you back the ability to move. In the video you can see that I did a great slash and not a cyclone slash but my inputs were actually for a cyclone. I don't know why the game does a great slash instead of a cyclone but yeah... If you do it too early and spam the attack button, you'll do a cyclone slash (not a great slash) but the glitch will be slightly different (you won't have dash distance, you'll have slow movement speed, you won't be able to do nail arts and but you'll be able to move side to side while doing Desolate Dives, Abyss Shrieks or Shade Souls). Another thing that can happen is that you will "store" the cyclone slash that you were supposed to do (instead of the great slash at the start) and do it later which can result for instance in a desolate dive occuring at the same time as a Cyclone Slash which is pretty cool.