Cable Crunches are a great exercise aimed at your upper-abs which is performed on the cable pull down machine. Primary muscles used in this exercise are the upper-abs. Secondary muscles used in this exercise are the lower-abs and the obliques.
To perform cable crunches you can use a rope attachment or short straight bar. Experiment with both and see what feels better for you. Grab the attachment and lower yourself down to the floor on your knees. With your hands above your head crunch down contracting your abdominal muscles. Rise back to the starting position in a controlled slower manner making sure that you keep the tension on your abs throughout the whole movement. Repeat the process.