Set the cable to the bottom of the rack and use either a bar, a rope or two handles. Hold the bar with hands at shoulder width apart and fully stretched.
Tuck your elbows into your abs and raise the bar until it wont move any more. At this point squeeze with all of your might for a moment.
Lower slowly and don’t stop till your arms are fully extended. In fact don’t stop at all, because when you reach the bottom you should immediately start the next rep.