Benefit
You get a +4 bonus on concentration checks made to cast a spell or use a spell-like ability while on the defensive or while you are grappling or pinned.
System Reference Document v3.5 © 2000–2003, Wizards of the Coast, Inc.; Authors Jonathan Tweet, Monte Cook, Skip Williams, Rich Baker, Andy Collins, David Noonan, Rich Redman, Bruce R. Cordell, John D. Rateliff, Thomas Reid, James Wyatt, based on original material by E. Gary Gygax and Dave Arneson.