Described as a unique alternative to the gutter downspout the rain chain is something that I have not come across before. Clearly they are popular because there are several videos on YouTube featuring rain chains. The video I have chosen is a commercial by HG Lifestyle, but it gives a demonstration of what it is and how it works. If you want to go the DIY route take a look at this site:
How to Make A Copper Rain Chain. Watch the video below and then choose between DIY or the store.