Variable field magnet motor?
That's clever! I have had it happen by accident occasionally, but never thought to use it for good effect.

It works by moving the rotor out of line with the stator so the effective magnet strength is lower. This increases Kv (velocity constant) because the motor has to run faster to produce enough back-emf to match the supply voltage. It is the permanent magnet equivalent of varying field current in a shunt-wound motor.

Motor speed could be raised by simply increasing voltage, but this also increases 'iron' losses due to hysteresis and eddy currents. Field weakening reduces the magnetic field strength as it raises rpm, reducing the loss at light loading. However it also reduces torque and output power, so it is more suited to applications which require high efficiency at both low rpm / high torque and high rpm / low torque.