It's a skill check. Specifically, it's the line of Counters that let you substitute a Concentration check for a saving throw.<br/><br/>BAB+Con? Sad for a 3/4 BAB class - quite sad, in fact...drops the bonus from +14 (10 ranks, +4 Con) to +9 (BAB +5, +4 Con). Now I have to decide if they're worth it or not anymore, since I do have a finite maneuver supply.