Magento Call custom action after “Save Config” in admin part

Open your module config.xml file and add this event.

<global>
        <events>
            <admin_system_config_changed_section_yoursectionname>
                <observers>
                    <mymodule>
                        <type>singleton</type>
                        <class>mymodule/observer</class>
                        <method>adminSystemConfigChangedSection</method>
                    </mymodule>
                </observers>
            </admin_system_config_changed_section_yoursectionname>
        </events>
    </global>

Next create file in Model/Observer.php in model folder and add code.

class My_Module_Model_Observer
{
    public function adminSystemConfigChangedSection()
    {
        echo "Test: oberver is working!";exit;
    }
} 

And clear cache and refresh browser ;)