The underhand (and narrow) grip pulldown better stimulates the lower - lat muscle fibers, giving the appearance of thick, full lats all the way down to the waist. (bodybuilding-wizard.com)
As with most external rotation exercises, stiff shoulders and tight chest muscles or tight lat muscles can limit range of motion. (all-about-arm-exercises.com)
Annette explains that toned lat muscles can create the appearance of a defined waist. (freedom-training.com)