Twisting the Knife- Upon scoring a critical hit in melee combat, the attacker may attempt a Combat Maneuver. If successful, they may on their next turn, roll an automatic hit, doing the weapon's damage plus any applicable bonuses. This action provokes an Attack of Opportunity from the opponent, both on the turns it is invoked, and the subsequent round in which it takes place.
You're effectively just initiating a Grapple check when you score a Critical Hit; the next turn, you deal damage with the knife or other light weapon as per the basic Grapple rules.
It's not bad but there's already this:
which is nearly exactly what you have, but a Feat. There is also:
And finally, there's this, which lets you attack twice during a Grapple (Move to maintain, Swift to Damage, Move to Damage):
Well, I don't like any of those. Mine is obviously better.