Interesting question, lets take a look at triceps anatomy. As the name implies, there are three heads, lateral, medial, and long. You'll notice the medial and lateral both attach directly to the humerus (upper arm bone), while the long head attaches to the scapula (shoulder blade).
So whats the significance? The medial and lateral head are involved in all elbow extension exercises, from chest press to over head presses to press downs, etc. The long head, which coincidentally comprises a huge portion of the upper is not fully recruited during these movements.
For the long head to be fully recruited, the shoulder must be flexed and horizontally adducted; iow, the lockout position of a close grip press. Then, elbow must be flexed and extended while maintaining this arm position.
So, the (one of the) most essential exercises for developing the triceps is the lying triceps extension, or skull crusher. Make sure you keep your upper arms fixed, however. This means elbows must not flare out, and no upper arm swing.
Br