You go to a web page without a form or prompt indicating it might double as a secret access to another web page. You go ahead and type a certain sequence of characters on your keyboard. If you get it right, you are redirected to a secret web page.
Something I've noticed since I started using non-IE browsers is that download instructions are provided, they're almost always specifically for the IE browser on Windows operating systems (right-click on the link then select "Save Target As..."). What about Firefox? Opera? Macintosh browsers?
Whenever you want to generate dynamic pages you often have to resort to using some server side scripting language such as PHP or ASP. But let us assume that there are only three or four pages that could be chosen from a drop-down menu (a combo-box).