They target the brachialis(sp?) muscle which is located on the outer portion of your arm between the bicep and the tricep. This is a lesser known muscle and often forgotten, however it will add size to your arms because when it grows it pushes both bicep and tricep outwards. Those with good brachialis development are immediately noticeable as you will have a nice bulge between the bicep and tricep. Those lacking development in this muscle are also noticeable as they won't have this extra thickness.