This is a wind powered cart that outruns the wind that propels it. There's a prop connected to the wheels, and as along as there's a wind (air moving with respect to the ground), then the wheels can drive the prop with sufficent thrust to slow down the wind even though the cart itself is moving faster than the wind, and outputting less power (more force but lesser speed) at the prop than it intakes at the wheels (less force but greater speed).

Spork & JB finally tested their human driven cart in natural wind on a dry lake bed.

http://www.youtube.com/watch?v=7SYvg40NHtc

http://www.youtube.com/watch?v=aDzWh9J1dk4#t=1m23s

http://www.youtube.com/watch?v=c5VlX-xEk00

Their blog:

http://www.fasterthanthewind.org/

Witchcraft!