The Zottman Curl is a very old exercise that is still effective now days, It was devised by strongman George Zottman in the 1880s.
The Zottman Curl allows you to add serious mass to your forearms while also building your biceps. This is especially good since most of us skip forearm training altogether.
When you curl the dumbbells up, the exercise primarily targets the biceps, with them being in a mechanically strong position when the underhand grip is used.
At the top of the exercise we rotate the wrists, switching to a overhand grip. As we lower the weight with the overhand grip, we readily engage the brachioradialis (a muscle which runs from the forearm to the upper arm) and brachialis (the muscle located beneath the biceps).
This allows you to target all the three major muscles that make up your biceps, and also hammer your forearms in one exercise.
Do not rock the body back an forth in an effort to lift the dumbbells. If you find it difficult, it means you are using weight that is too heavy.