The forearm muscles tend to be weak and this is a good exercise to strengthen them. It is better to have the back of the bench up rather that sit free, since more effort can be put into moving the dumbbells and less into stabilising the torso. Another way of putting it is that all arm work is difficult since it is very easy to cheat and slightly swaying back as the dumbbells rise is a popular way of being able to lift a heavier weight, so pleasing to the ego – but leads to less progress in arm strength or bulk, depending upon the goal of the session.
For best results, leave the ego in the changing room and concentrate on moving from the elbow only; shoulders and back are still. Turn the palms downwards as much as possible on the lowering phase and squeeze the dumbbell hard, keeping the wrists strong. Lower the dumbbells more slowly than you lift them.
Treat the forearms with respect – it is very easy to injure them through over enthusiasm. It is safer to stop well short of failure with this muscle group. How do you think I know this?