I'm confused on this point. This seems similar to the description of an oberth manuever, in which a craft derives extra kinetic energy at infinity from its propellant than would otherwise be possible, by burning its propellant while the propellant's kinetic energy is at a maximum with respect to a gravity well.otherwise we'd have a paradox where converting chemical energy into electricity and then a magnetic field would change the mass of the system at infinity which is impossible.
"At very high speeds the mechanical power imparted to the rocket can exceed the total power liberated in the combustion of the propellant; this may also seem to violate conservation of energy. But the propellants in a fast-moving rocket carry energy not only chemically, but also in their own kinetic energy, which at speeds above a few kilometres per second exceed the chemical component. When these propellants are burned, some of this kinetic energy is transferred to the rocket along with the chemical energy released by burning."