You press the thumb against the index finger.

Pulling the thumb back toward the wrist is just retarded.

Do you see guys doing a phoenix-eye fist without a thumb brace? No, you don't.

Doing it without a thumb brace is a good way to **** up your knuckle AND your wrist.

CLF schools that teach a panther fist without a thumb brace should be ashamed they are teaching students a good way to ruin their fist/fingers/wrist/hand.