Taste for blood giving 3 procs on 21 sec window when the enemy does not have an absorb effect active.
However when a paladin for example got sacred shield up, it might not proc at all during that 21 seconds.
It’s in WotLK, I’m getting it on my Warrior (lvl 26 atm). It’s a 3 point talent that has a 33%/ 66%/ 100% chance to make Overpower available for use when Rend ticks (with a “cooldown”, IIRC the effect cannot occur more than once every 7 seconds or something). It seems - based on OP’s post - that the talent does not proc if the damage from the Rend tick is absorbed by a shield or something (I haven’t experienced this quite yet as my warrior is very low level and doesn’t PvP), and he’s wondering if it’s by design.