code: function ( model ) {
                  var trailing = this.collection.trailing && this.collection.trailing.id || this.collection.first().id,
                      view = this,
  
                      cbUpdateTrailing = function () {
                          view.collection.get( Math.round( this.trailingId ) ).select( { label: "trailing" } );
                      };
  
                  // Animate the trailing element and make it catch up. Use the .select() method with the label
                  // "trailing".
                  $( { trailingId: trailing } ).animate(
                      { trailingId: model.id },
                      { step: cbUpdateTrailing, done: cbUpdateTrailing, duration: trailingLag }
                  );
              }
docs: Only runs when a model is selected with the default label, not with the custom label "trailing"