Muscle Group: Biceps
You can do Zottman Curls two ways, by either taking two dumbbells and sitting on the edge of a flat bench or by standing. Let your arms hang straight down, with your palms facing forward.
On normal wrist curls, you would flex the wrist upward or at least have a tendency to do so. The action here will be opposite of this, you will literally bend your wrist backwards.
Now, after you curl the dumbbell all the way up, straighten the wrist and turn your forearm so that your palm is now pointing downwards as if you were doing a reverse dumbbell curl. Slowly lower the dumbbells, taxing both your forearms and biceps.