Back To The Future may be my all time favorite series of movies. Ever since BTTF2, I've been patiently waiting for scientists to create a hoverboard but for some reason we still aren't there yet. When they filmed this iconic scene, CGI still was in its infancy and so good old fashion ropes and harnesses were used. In this video you can see this rig in its testing phase.
Behind the Scenes of Back To The Future Hoverboarding