Muscle Group: Forearms
I personally feel that this exercise is not incorporated nearly enough in the bodybuilding community. Forearm Twists are one of the greatest mass builders for the extensors known to man. Forearm Twists specifically target the brachioradialis. This is the most prominent of the forearm muscles. When the palm is down, the brachioradialis is the muscle that dramatically pops up and over your elbow joint.
Hold a dumbbell with your forearm resting on your thigh. Then twist the weight as far to the right as you can and then as far back to the left as you can.
Forearms are best trained with biceps because it is impossible to get a great bicep workout without placing a lot of stress on the forearms.