The best I can do to explain this is youtube comments:
Slightly differently worded explanation:The inner rotor and shaft you see out to the right is hooked up to whatever it is that this motor is running, and here we see it being started under load. In order not to fry the motor components from the initial shock of trying to get a bunch of machinery started from a standstill (too many amps), the ring of electromagnets is allowed to rotate freely when the power is initially turned on. The operator is winding down a mechanical brake which slowly stops the outer half of the motor as power is gradually transferred over, and as soon as the load is up to speed the motor can be locked down.
Other than that, it's just another basic electric motor. Still pretty cool if ya ask me!
I watched the whole video and see what's going on now. What starts as the rotor ends up as the stator, that is, the roles change after initial spinup. This helps in starting a large load, where at first the load side is held stationary and the other side is allowed to spin up to build up some kinetic energy. After the kinetic energy is built up the load side brake is released and both sides of the motor are coupled together to transfer the kinetic energy to start the load. After the kinetic energy is mostly used up, the non-load side (the rotor that is now becoming the stator) is fixed to mechanical ground to continue providing torque to the load. I imagine that's what that crank the guy was turning is doing, changing the brakes and clutches.