A class loader method.
The _ method of JHtml is a static function that grants quick access to a number of other static methods in the Joomla! platform's HTML subpackage, in particular those classes with the JHtml prefix (i.e. JHtmlBehavior, JHtmlGrid). The return value of this function depends on the return value of the method resolved by parsing the first parameter.
JHtml::_() expects its first parameter to be a string of period-separated values representing a class prefix (optional), a file name, and a method, in that order. When only a filename and method are provided, as seen in the first example below, the prefix defaults to "JHtml" and the resolved class name will consist of "JHtml<filename>." The filename segment of $key is used to find the file in which the class and method are defined among whatever include paths have been attached to the static JHtml class using JHtml::addIncludePath. The "/libraries/joomla/html/html" folder is attached to the JHtml class by default when "/libraries/joomla/html/html.php" is included, but other paths can be added, making the functionality of _ extensible.
The above code will output the return value of JHtmlForm::token, found in "/libraries/joomla/html/html/form.php," (which happens to be a hidden form element containing an anti-spoofing token.)
The above code will output the return value of a static method, myExtensionButton::push(), which should be defined in a file called "button.php," provided that this file exists in the default include path.
Any additinal parameters passed to JHtml::_() will be passed as parameters themselves to the resolved method.
Adding many include paths to JHtml::_ could negatively impact its performance, since it has to search through each of those paths in order to find the method that matches the $key string. However, once a particular key has been used to invoke a method, that key and its corresponding  array are cached. As a result, any future calls using that key will execute much faster.
public static function _ ($key)