Twig extension method argument which is not specified in twig filter

This code is from the symfony Knp-paginator-bundle and lives in a Twig Extension class (PaginationExtension.php). I'm wondering (just curiosity) where the second argument: SlidingPagination $pagination in the render() method comes from, since this argument is not supplied in twig. Anyone an idea?

/**
 * {@inheritDoc}
 */
public function getFunctions()
{
    return array(
        new \Twig_SimpleFunction('knp_pagination_render', array($this, 'render'), array('is_safe' => array('html'), 'needs_environment' => true))
    );
}

/**
 * Renders the pagination template
 */
public function render(\Twig_Environment $env, SlidingPagination $pagination, $template = null, array $queryParams = array(), array $viewParams = array())
{
    return $env->render(
        $template ?: $pagination->getTemplate(),
        $this->processor->render($pagination, $queryParams, $viewParams)
    );
}

In Twig the extension is used like this:

{{ knp_pagination_render(projects) }}

Actually SlidingPagination $pagination comes from projects

in twig call: {{ knp_pagination_render(projects) }}

\Twig_Environment $env is inserted to all extension functions by twig.