It's ability block that can't be dispelled and with Sith Assassin you can make go before anything else. That is huge because the biggest problem for Sith teams was the triple cleanse squad. Now they can't cleanse until round 2. It's over by then. And if it's a team that is tanky enough to withstand that then they are going to run into annihilate. Or the 2 turn EP stun on multiple shocked toons. Or a Maul that has 45% evade and has racked up a ton of extra health. It's not unbeatable but its one of if not the best squad out there.