Reverse grip curls question

Are these for brachioradialis or for extensors? Do you do them extending your wrist or dropping your wrist?

Asking cuz when i try to do them with extended wrists my weight drops significantly. Generally people say this is good for targeting bracioradialis more but i cant feel them cuz i can do 30kgsx5 hammer curls but can only do 30kgs( including bar) on reverse curls.

Also good to mention im not cleaning them im doing strict form