You channel your ki energy to splinter the defenses of creatures whose tough hides or magical natures normally allow them to shrug off your blows.
Prerequisite: Dex 13, Wis 13, Improved Unarmed Strike, Stunning Fist, base attack bonus +9.
Benefit: You gain a +4 bonus on damage rolls when you make an unarmed strike against an opponent whose damage reduction is effective against your unarmed strikes. This feat’s benefit does not apply against a target if its damage reduction does not apply to your attacks. For example, a monk with ki strike (adamantine) does not gain the bonus on damage rolls against a foe with damage reduction that affects attacks with adamantine weapons.
Special: A fighter can select Water Splitting Stone as one of his fighter bonus feats. A monk with the Stunning Fist feat can select Water Splitting Stone as long as she has a base attack bonus +9 (other prerequisites can be ignored).
Source: Player’s Handbook II, page 85.