Originally Posted by
Melange
'Deathing' something is Priest lingo for using Shadow Word: Death's backlash damage to break incoming CC. There's a small delay when you cast it before the backlash damage hits, which means that with precise enough timing, you can cast it right before being CC'd, so the backlash damage breaks the CC. This is most easily done with predictable CCs (Polymorph), but if you can read your opponents, you can do it with any CC that breaks on damage.
That video has Hydra Deathing a rogue's Blind, which (as you probably know) is an instant cast.