code: function () {
              var carousel = this,
                  newpos   = 0,
                  selected = carousel.get("selectedItem");
  
              if (selected == this._firstItem) {
                  newpos = selected - carousel.get("numVisible");
                  carousel._selectedItem = carousel._getSelectedItem(selected-1);
                  carousel.scrollTo(newpos);
              } else {
                  newpos = carousel.get("selectedItem") -
                           carousel.get("scrollIncrement");
                  carousel.set("selectedItem",carousel._getSelectedItem(newpos));
              }
          }
docs: Select the previous item in the Carousel.
  
  @method selectPreviousItem
  @public