You have a greater chance than normal to resist attacks against your vitality, but at a cost.
Prerequisite: Great Fortitude.
Benefit: Before rolling a Fortitude save, you can decide to activate this feat, which gives you a +6 bonus on your saving throw. After the saving throw is resolved, regardless of the result, you are staggered until the end of the encounter.
Source: Savage Species, page 31. Removed the last sentence and hyperlinked to the version 3.5 definition of the staggered condition.