Snapping Turtle Shell
Your guarding hand is almost magical in its skill at deflecting incoming blows.
Prerequisite: Snapping Turtle Clutch, Snapping Turtle Style, Improved Grapple, Improved Unarmed Strike, base attack bonus +5 or monk level 5th.
Benefit: While you are using the Snapping Turtle Style feat, the shield bonus the style grants to your AC increases to +2, and your enemies take a –4 penalty on critical confirmation rolls against you.
Source: Ultimate Combat, p. 120