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does anyone know how it works that towards the end of the video, it shows the resulting torque curve of the motor is actually negative for a decent span. Does this mean the motor actually resists rotation for that arc during every rotation?

Wouldn't that be a significant hit to efficiency, or do the benefits simply outweigh it for a net gain?

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I think the idea is that the inverter's job is to always be continually changing the current to the motor's stator windings such that you'd always have close to the optimal angle between the rotor and the magnetic fields created by the stator. Kind of like how a gas engine might backfire if you fired a spark plug at the wrong time so a well designed engine would avoid doing that.

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