It works well on longer casting time skills, but it takes about a full second before it hits. Distracting blow is better for interrupting, but no extra damage against spells.
You can get this skill from the quest, "The Royal Papers" at Fisherman's Haven. It can also be purchased from the skills traders at Droknar's Forge and Ember Light Camp.
This skill got nerfed to stop bot farming outside Droknar's. Sad. It now hits quicker (gotta stop that Shield of Judgement cast!), but costs 10 energy and has a 20 second recharge, making it useless for serious warriors who are already energy-starved. This was totally unnecessary. Anet could have: 1) given the Braves Distracting Blow, or 2) given the Fierce Savage or Distracting Shot. There was no real need to ruin a decent skill for this purpose. Not like it was overpowered, considering that it was seldom used as it was. Seldom just became never.