Punishing Blows (3.5e Feat)
From D&D Wiki
Punishing Blows [Ki Manipulation]
You have discovered how to deliver Unarmed Strikes that truly weaken your opponent
Benefit: You have learned to deliver unarmed strikes that truly weaken your opponent. When you use this feat pay 2 Ki points to force them to make a fortitude save (DC: 10 + 1/2 character level + Str Modifier) On a failed save your opponent takes 2 Temporary Con Damage.