There are basically the same cable exercise, executed with a different hand grip. But, oh! What a difference the grip makes! Putting these two together will give you a far more complete triceps workout than either one alone. Here's what it looks like:
Attach a bent bar to the upper connection of a cable machine. Stand in front of the cable machine with your legs in a slight scissor position so that one leg is stepped slightly forward and the other leg is stepped slightly back. Hold the bar up at chest level in both hands with your palms down, and your pinkies are on the outside bend of the bar. Your elbows should be bent and held in at your sides. Use your triceps to push the bar down toward your thighs. Keep your elbows in close to your body as you descend to concentrate the work in the triceps. When your arms are fully extended and the bar is down in front of your thighs, reverse motion and bring the bar back up to starting position.
So far, so good. Here are the strengths of the tricep press-downs:
Work all heads: Your triceps get their name from the three heads (lateral, long, and medial) into which the muscle forks at the bottom. The trick of tricep work is, first and foremost, to hit all three heads of the muscle. Press-downs are great for this, as this exercise spreads the work thoroughly among all three. It also targets your anacondas (just on the outside of your elbow).
Vary your attachments: You can keep the overall benefits of the push-down while shifting the exercise's focus by replacing the bar with a double rope attachment. The variation using the rope will shift the intensity to the lateral head (outside aspect) of the triceps, where definition will definitely be noticed.
Clearly, tricep press-downs are a thorough and balanced exercise with room for variation. They are an essential part of any comprehensive triceps program, and an absolute cable-machine standard. Still, just by turning your hands over you can get additional benefits—as long as you are careful about your limits.
To do pull-downs, take up the same staggered position as for the previous exercise, but flip your hands over so that they are palms up on the bar, positioned so that your pinkies are resting in each bend of the bar. Your elbows should be bent and held in at your sides. Use your triceps to pull the bar down toward your thighs. Keep your elbows in close to your body as you descend to concentrate the work in the triceps. When your arms are fully extended and the bar is down in front of your thighs, reverse motion and bring the bar back up to starting position.
Here are the benefits of tricep pull-downs:
Engage the arm: The palms-up grip used in this exercise shifts the focus to the medial head (inner, lower aspect) of the triceps and increases the intensity to both the other muscles of both the upper arm and the forearm: extensor carpi ulnaris, extensor digitorum, extensor digiti minimi, and the extensors carpi radialis longus and brevis. That's a lot of Latin to tell you that this exercise will make a connection between the upper arm and the muscles running along the forearm toward the wrist. Since a lot of guys are looking for ways to develop overall arm (including forearm) strength, this exercise can be a great two-fer.
Watch your weights: The movement of exertion to the lower and inner aspect of the triceps and to the weaker forearm muscles will preclude working heavier weights, but the results will be more development in the upper and lower arm generally and an increase in the development of the lower triceps area specifically.
In short, if you want to really isolate your triceps, and hit all three heads of the muscle fairly evenly, you go with a push-down. To hit the outer head of the muscle, use a rope attachment and do a push-down. To hit the inner and lower head (with secondary benefits for upper and forearm strength), use a pull-down—but plan on using less weight.