Start: Hold two dumbbells with an underhand grip and kneel o the floor, facing the long side of a bench. Position your forearms on the bench so the dumbbells hang off the other side. Open your palms to let the weights roll down to your fingertips.
Finish: Roll the weights back up as you close your fingers and curl your wrists up.
It's just as effective to do a set with one hand and then with the other. This is also useful for evening out muscle size and strength imbalances.