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I suspect this is not your question.
.
Maybe he is refering to a motor connected to a VFD, and the motor slows down due to a decrease in the input frequency.
A motor can be represented by a voltage source in series with the stator winding resistance, the stator winding inductive reactance, and the counter EMF. (There might be some other items in this circuit, depending on how exact you want to be).
Anyhow, The counter EMF opposes the applied voltage. It is determined by the rotor properties. Its magnitude is some constant multiplied by the speed of the motor.
Reduce the speed of the motor, and you reduce the counter EMF. With a smaller Counter EMF, you have more voltage across the stator winding. Since the stator winding is mostly inducatance (the resistance is very small), there is more inductive current. So the reactive power goes up.
Anyhow, that's the basic idea as I remember it from the simple motor's equivalent circuit. I'm probably leaving out some details, that may or may not be important.