Attach a straight bar to a high pulley. Grab the bar a bit wider then shoulder width. Sit or kneel in front of the pull down machine.
Straighten your arms and let your lats stretch. Hold this position for a second. Arch your back slightly and pull the bar down in front of your head until the bar is just below your chin. Hold this position for a second to maximize the peak contraction. Slowly straighten your arms to the starting position. Repeat.
Tips - you can vary your grip to work your back from different angles. (i.e. wide grip, narrow grip, overhand, underhand, etc.)
source: www.leehayward.com
