Using the underhand grip on a band or cable helps tap into more corrective-based ranges of motion for the shoulder and upper back while learning to stabilize the thoracic spine and shoulder blade during dynamic rotational movements at the true shoulder joint itself.
This variation requires lighter loads and slower, more controlled movements in order to avoid flaring up the shoulders or placing undue stress over the rotator cuff or other acute muscles being loaded. Set your ego aside and load this movement properly in the 6-12 rep range with total control and stability. The activation benefits will become readily apparent.