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How?

By offsetting the mass of a system, to produce a zero mass vehicle.

Very cool stuff

Take a net negative mass vehicle, and accelerate it by having it collide with its propellant. Would this be an anti-rocket?

How does it come to a stop, the vehicle gains too much mass?

The mass part of the vehicle could either detach from the negative mass component. Or the vehicle could eject mass to reduce velocity.