You can follow up on a particularly powerful unarmed attack with a mighty kick, spinning in a complete circle before landing the kick.
Prerequisites: Str 15, Improved Unarmed Strike, Power Attack.
Benefit: If you score a critical hit on an unarmed attack, you can immediately make an additional unarmed attack against the same opponent, using the same attack bonus that you used for the critical hit roll. For example, a 15th-level monk can make three unarmed attacks in a round, at base attack bonuses of +11, +6, and +1. If she scores a critical hit on her second attack, she can make an additional attack using her +6 base attack bonus. She then makes her third attack (at +l) as normal.
Source: Complete Warrior, page 105.