The same principal works here as with curls, except in the opposite direction. As a result, the other side (the elbow side) of your forearm is worked. As with the curls, leverage is generated by the weight being positioned far away from your hands, way out on the lever bar. This leverage attempts to make your wrist rotate down toward the ground, which will cause the lever bar to drop out of alignment with your forearms. By resisting this downward force, and holding the lever bar in a straight line with your forearms throughout the movement, you will get a GREAT forearm workout.
By doing both curls and tricep extensions with the Hammer Curl Bar, you will hit your forearms from both directions. As usual, the photographs were taken with no plates on the bar so that your view of the proper bar position would not be obscured.