A LOT of warriors do not know this. If you are speced into Unrelenting Assault when you overpower someone while they are casting they get a debuff which reduces their magical dmg and healing by 50% and lasts 6 secs. THIS STACKS WITH MORTAL STRIKE.
Reduces the cooldown of your Overpower and Revenge abilities by 4 secs and increases the damage done by both abilities by 20%. In addition, if you strike a player with Overpower while they are casting, their magical damage and healing will be reduced by 50% for 6 sec.
It doesnt make them heal for nothing but it made a hateful geared shaman Healing Wave for about 300 around there.