I should have added a description here insead of just on youtube lol.
so for this to work you have to cast a spell first.
while the animation is going you attempt to pick up the item and immediately open your inventory. if done correctly the item should be on your cursor with the inventory open as you see in the vid.
if youre having trouble with the timing it may be because your animation is too fast (faster cast rate/increased attack speed). try it without any fcr/ias gear first as it will be much easier to find the slower timing.
also, you know your timing is too fast when u attempt to pick up the item, but the item itself doesnt jump off the ground.