code: function(cmp) {
          var me = this,
              newTab = cmp,
              oldTab;
  
          if(Ext.isString(cmp)) {
              newTab = Ext.getCmp(newTab);
          }
  
          if (newTab === me.activeTab) {
              return false;
          }
  
          oldTab = me.activeTab;
          if (me.fireEvent('beforetabchange', me, newTab, oldTab) !== false) {
               me.activeTab = newTab;
               if (me.rendered) {
                   me.down('container[baseCls=' + Ext.baseCSSPrefix + 'grouptabcontainer' + ']').getLayout().setActiveItem(newTab);
               }
               me.fireEvent('tabchange', me, newTab, oldTab);
           }
           return true;
      }
docs: Makes the given component active (makes it the visible card in the GroupTabPanel's CardLayout)
  @param {Ext.Component} cmp The component to make active