Cable reverse-grip pushdown

Attach a straight bar to a high pulley cable machine. Stand facing the machine with your feet shoulder-width apart. Grasp the bar with an underhand grip, palms facing up, and your hands shoulder-width apart. Keep your elbows close to your sides and your upper arms stationary throughout the exercise. Using your triceps, push the bar down until your arms are fully extended and your triceps are contracted. Pause for a moment, then slowly return the bar to the starting position. Repeat for the desired number of repetitions.