Much lighter and simpler than a driveshaft.
http://www.bikeforest.com/hularent.html
From the website:
"The hula bike is another hand-built creation from the Bicycle Forest. Instead of a conventional drivetrain, the hula bike uses an eccentrically laced rear wheel. Because the hub is offset from the centre of the rim, the bike can be propelled by hopping up and down with the right rhythm. "