I read a Frederic Delavier’s book « Strength Training Anatomy » and there is good stuff.
Standing in front of the machine with your hand on the bar with a supinated grip. Your elbows are along of your body. Your arm must be do a « L » :
Inhale and make an extension of your arms and be sure not to spread your elbows of your body.
Exhale at the end of the movement
The supinated grip doesn’t allow you to work with heavy weights then this exercice is done with light weights.
This exercise work triceps and the medial head.
When you do the extension of your arms, you work also anconeus muscle and wrist’s extensors.
The wrist’s exensors ( extensor carpi ulnaris, extensor digitorum, extensor digiti minimi, extensor carpi radialis longus and extensor carpi radialis brevis) keep your wrist straight by an isometric contraction during the movement’s execution.