Muscle control: Forearm Roll
Rarity: Extremely rare
Due to their physiology, forearms are incredibly difficult to control. They are documented (and expected) only to roll.
Most performances are triggered by auxiliary slight movements of the fingers, and so they probably are---at most cases---a pseudo muscle control.
Our Youtube playlist of this control may be found here.
Diesel Danny, a performer on Youtube circa 2014, utilized forearm roll during his flex sessions quite often.
Another performer known to perform this muscle control, Sergio Carpathos.