Hexdrinker confusion

So by now this has happened to me all too much. Me being a AP main, and AP champs being knows for 1 to 3 spell kills on squishies, I just dont understand Hexdrinker as an item. Very often I roll into a carry's face with a spell removing about most of the carry's hp and then on my next spell which i expect it to kill them (Typically FIzz with Lichbane & W on) hexdrinker pops and the shield stays as full as full gets and they take about 0 damage. I simply dont understand, does it negate the spell that brings you down to 30%? If so, then why isn't that is the description?