Depending on your preference, the seated barbell wrist curl could be performed by either placing your forearms on your knees or on the bench. Forearms should be placed at shoulder width.  The composition of your arm will determine the width of the grip that will be most comfortable for you – take a narrower or a wider grip accordingly.  Holding the barbell with the thumb on the same side as the rest of the fingers may help you extend the range of motion.  Placing the thumb over the bar and opposite to your fingers may feel awkward and cause pain in the wrist.

The wrist curl motion itself is very simple, flex  on the way up and relax your forearm on the way down, breathing out on contraction and in on the relaxation.  You do, however, have choices on how you bring the weight down; you can either keep your fingers in a fist over the bar, or extend your fingers and allow the bar roll down to their tips (then bringing the weight back up with your fingers into a fist and lifting it for a full contraction).  This is a great alternative for extending the limited range of motion when using light weight (it probably won’t work very well when you decide to go heavy).  You should control both the contraction and the relaxation of the flexors.  Restrain yourself from swinging the weight to keep good form. 

If you choose to perform the seated barbell wrist curls on your knees, avoid cheating by bouncing your hand up with the knee.  Keep your wrists completely removed from the support (bench or knees) to allow the greatest range of motion and to keep the pressure on the flexors throughout the exercise.

The seated barbell wrist curls target the forearm flexors.