I often refer to this exercise as the power push.
Grip a straight lat-pulley bar with hands placed 12 inches (30 cm) apart. Bend the knees to assist with your body.
Keeping your elbows wide (not tight against the side of your waist), press the bar downwards. Return to starting position and repeat.
Remember to start from under the pec line – not higher – and you will build size in both the middle of the triceps and in the lower-arm regions, because the elbows are held high and wide as you power down against the resistance.
(Actually, in my gym, there is a convenient post opposite the lat pulley, affording the exercisers the added advantage of being able to lean against this support for balance.)