Flapping wing vehicles achieve lift by moving a wing back and forth through the surrounding medium, much like a bird or insect does.
There are two distinctly different forms of flapping wings - bird wings and insect wings. Birds derive most of their lift by pushing their wings downward. This is called the downstroke. At the end of the downstroke, the bird turns its wings to reduce drag and then pulls them back to the beginning of the downstroke. This is called the upstroke. During the upstroke, the bird actually pulls itself downward because of aerodynamic drag forces on the wings. As a result, birds are not very stable. Insects derive their lift by pushing their wings side-to-side. This way, the wings produce lift during both strokes. Effectively, the wings are pulled forward at some angle of attack, generating lift. At the end of the stroke, the wings flip back on themselves and are pulled backward at the same angle of attack, generating the same lift.