An oscillation thruster (also known as a stiction drive, internal drive or slip-stick drive) uses the motion of internal masses to create a net thrust. These thrusters include either vibrational or rotating masses, in which one portion of the cyclical motion is high-speed, and the other low-speed, or alternately high and low impulse. The result is that for some of the motion there is a high force being generated, enough to overcome friction. However on the "return stroke" the force is not high enough, and any motion occurring in the first portion is not reset. In this way the devices "steal" working mass from their supporting surface, a fact that may not be apparent to casual observation.
in se
While such a device may not necessarily violate the law of conservation of energy, it would appear to violate conservation of momentum, a fundamental principle of all current understandings of physics, and is therefore considered by most physicists impossible to construct outside of science fiction.
All devices that have been claimed to be reactionless drives and tested under controlled conditions have failed
and are found to rely on a non-linear effect of the supports they sit on. Some examples include devices that inch along a tabletop or when floating in water, but stop working once they are suspended or in vacuum.