A conductor exposed to a time varying magnetic field will move
current through the conductor. The conductor in this case is the
tether or a wire sheathed by it. The magnetic field the wire sees
changes because of its orbital motion through the varying topology
of the Magnetosphere. You can use this to deorbit an orbiting
object by disipating kinetic energy as heat produced by electrical resistance in the cable or maybe in this case providing power to
the spacecraft.

Hmmm...

"During the flight, the process of collecting energy will begin when the electromagnetic portion of the tether collects electrical current along the tether's length as it moves through the Earth's magnetic field."

The current produced in the wire moves electrons into the spacecraft and leaving the wire with a positive charge that attracts more electrons from the surrounding environment. You wouldn't think that they could
move alot of current with LEO as a "ground".

"To keep the current flowing, the plasma contactor reconnects the electrons with the invisible, electrically charged plasma that surrounds the Earth, emitting the electrons back into space so it can complete its circuit."

Then to remain electrically neutral (or close the circuit) it uses an electron gun to dump extra charge.