Plastic Bottles Turn to Roofing Materials in Disaster Zones

I wish somebody would explain this to me. This is the second time I've seen it and I don't get it... At at all! How does this work again?

http://www.triplepundit.com/2012/11/nyit-turns-kickstarter-green-technology/

It looks like it will stop about 90 % of the water if it is applied on a 5/12 pr 6/16 pitched roof. A good wind might do a lot of harm too.