This exercise can be performed using a straight bar or rope attachment. Personally I prefer the straight bar. Experiment to see what feels better for you.

 

To perform cable push downs grip the bar slightly narrower than shoulder width. Lean forward into the exercise and press the weight down to your waistline. Control the weight back up to just under your chin, roughly 45 degrees past the parallel plane of your forearms. Drive the weight back down to your waistline, repeating the process.

 

By going 45 degrees past the parallel plane you minimise the stress placed on your elbows.