Warning: Wikipedia contains spoilers
The trick is dependent on restricting the audience's point-of-view and being aware of the body's angle with respect to the viewer(s). The illusionist is simply standing on the tip-toe of the foot furthest from the audience; that foot is concealed from view of the audience by the foot nearest to the audience and by the illusionist's clothing.
It is a tricky illusion to perform because of the physical strength required and the precision in orientation. Obviously, it cannot be performed in front of a large, spread-out crowd, or sustained for any real length of time.
David Blaine performs this trick; on television his act is intercut with shots of him suspended by wires taken at a different time so he can be shown with both feet above the ground in certain shots.