A full range of motion is key to getting the most benefit out of this exercise. Brace your elbows on a flat bench, and lower the bar until the lever part of the bar is pointing straight down. Do this by letting the wrists drop, do not lower the entire forearm. Then, slowly and deliberately rotate your wrists upward as high as they will go (no swinging motion should be used; do not use momentum to raise the bar). Try to squeeze the bar even higher and hold this top position for a few seconds. Keep your elbows on the bench, but your forearms may raise up off the bench an inch or two. We recommend higher reps (10 - 20), because it is too easy to deviate from the proper form when handling heavy weights.
For an even nastier set, when you reach failure on the wrist curls, just stand up and continue doing full arm hammer curls with the same bar and weight, bending at the elbow and keeping your wrists locked in a straight position (see "Exercise: Basic Curls with the Hammer Curl Bar")