Onslaught

Combat

No one is prepared for how hard you strike until they see it firsthand.

Prerequisite: Str 13, Power Attack, base attack bonus +1, sneak attack class feature.

Benefit: If you use Power Attack with the first melee attack you make during a surprise round, you can sneak attack even if you are not flanking your target and it’s not denied its Dexterity bonus to AC.

Source: Advanced Class Origins