Using a close grip with supinated hands is easier than using a wider grip in pronation; however, the biceps work more and the back works less with this version. (bodybuilding-wizard.com)
This happens because the narrow grip forces the biceps into a more supinated position than wider grips (supination is the movement where the hands rotate to a palms - up / forward position). (muscle-fitness-training.com)
Lusk et al. (2010) compared the lat pull - down performed with a pronated and supinated hand - grip on latissimus dorsi muscle activity. (strengthandconditioningresearch.com)