As a web developer i have seen these HYIP scripts in action. All HYIP schemes are considered ponzi schemes and they work as long as cash is flowing in the system meaning as long as new users are coming into the system and pouring cash into the system other users who signed up earlier in the system will be getting paid.

So when the cash flow stop the hyip site will stop paying its users. Actually there is a site that list currently active HYIP programs and it also list latest scam hyip sites as well. You can check it out here : http://allmonitors.net/scam/