This exercise is a very good one for developing both the size and shape of the triceps muscles. 

1 Position your feet in a split stance - one foot in front of the other with your feet hip-distance apart - facing away from the cable machine. Take an end of the rope in each hand and position it behind your head, with your elbows pointing upward.

2 Exhale and slowly extend your arms at the elbow while maintaining your body position - your arms should be parallel with each other and your elbows should point up. 

3 Keep extending your arms until they are straight, then inhale and slowly lower your arms to return to the starting position.