code: function (collection, settings, elements, element, index) {
              if (index !== 0 && trueOrUndefined(settings.before_up(collection, element))) {
                  elements = swapElements(collection, elements, index, index - 1);
                  if (!trueOrUndefined(settings.after_up(collection, element))) {
                      elements = swapElements(collection, elements, index - 1, index);
                  }
              }
  
              if (settings.position_field_selector) {
                  return doRewritePositions(settings, elements);
              }
  
              return elements;
          }
docs: reverse current element and the previous one (so the current element appears one place higher)