An electric motor and could also be used as generator,
being an accessary on jet engine or rocket engine's turbo pump (possibly a career tech node),
converting shaft power into electricity
or consume electric power to drive the turbomachinery in order to get quick throttle response.
The power of this motor could be set in designer UI,
increasing this power would provide more electric power and further reduce spool time
but increase weight, cost and reduce engine thrust due to extra energy extracted by turbine.