Moving a wire in a magnetic field produces a voltage between the ends of the wire (and a current in the wire if there's any conductor between the ends of the wire).
If the direction of the motion is exactly reversed, then the voltage (and the current) become negative but with the same size.
By changing the direction of the motion, the size of the voltage (and current) can be changed to anything between zero and the maximum. It's zero when the wire moves parallel to the magnetic field, and maximum when the wire moves perpendicular to the magnetic field.
