The straight arm lat pull-down is an exercise that is a commonly employed exercise that is great for building the lats ("wing muscles"). However, the traditional standing exercise places a great deal of stress on the shoulder tendons and joints, as well as sometimes being hard on the back. This variation, done on a ball or a mobile bench, allows you to work this area in a dynamic and full range of motion while being safe on the shoulder/low back. It can actually be beneficial to improve your range of motion if you're tight here. A key part of keeping this exercise safe for the back is to engage the abdominal muscles just enough so that your ribs do not flare upwards as you reach above the head. NOTE: due to the large range of motion, you will need to use lighter weights or bands than you would for other lat exercises.
Enjoy and be active!
Kirk Iodice, DC, ART