Wrist curls are used to strengthen the forearms. Forearms are a vital piece of most strength training exercises; however, they are neglected by most lifters.
Begin by sitting on a flat bench with a barbell across your thighs. Now grip the barbell with your palms up and your hands at a close grip. Now, place your forearms flat on the bench and in between your legs. You want your hand to be extended beyond the bench and only your forearms should be supported by the bench. Begin the movement by curling your wrists up and squeezing your forearm muscles. Squeeze the contraction at the top and then lower your wrists back to the starting position. Repeat this movement until your set is complete.
Use light weight for this exercise and most people do high reps. Even though this is only a short range of motion, you should really feel the movement throughout your entire forearm.
This exercise can also be done with dumbbells and performed the same way. You can also rest your forearms on your thighs, rather than in between your legs; this may be more comfortable for some lifters.