Note that when you clicked on the button, the form actually posted back to itself. However, the state of the form was remembered from one webpage view to the next. This is known as FormState.

QForm objects, in fact, are stateful objects that maintain its state from one post to the next.

In this example, we have an $intCounter defined in the form. And basically, whenever you click on the button, we will increment $intCounter by one. Note that the HTML template file is displaying $intCounter directly via a standard PHP print statement.

Also note that session variables, cookies, etc. are not being used here -- only FormState. In fact, you can get an idea if you do View Source... in your browser of the HTML on this page. You will see a bunch of cryptic letters and numbers for the Qform__FormState hidden variable. Those letters and numbers actually represent the serialized version of this QForm object.