code: protected function getFrontendFilterValue($arrWidget, $arrFilterUrl, $strKeyOption)
      {
          // Toggle if active.
          if ($this->isActiveFrontendFilterValue($arrWidget, $arrFilterUrl, $strKeyOption)) {
              return '';
          }
          return $strKeyOption;
      }
docs: Translate an option to a proper url value to be used in the filter url.
  
  Overriding this method allows to toggle the value in the url in addition to extract
  or inject a value into an "combined" filter url parameter (like tags i.e.)
  
  @param array  $arrWidget    The widget information.
  
  @param array  $arrFilterUrl The filter url parameters to use.
  
  @param string $strKeyOption The option value to determine.
  
  @return string The filter url value to use for link gererating.