T-Slow, I think you are basically correct. But remember that happens only in the fibers that fire (i.e. that are triggered by the EMS). So you have go high enough with current intensity.
In addition you cause more muscle fatigue in those fibers, than you would with voluntary efforts. Finally, EMS done for a sufficient number of weeks would convert some fibers I and IIx fibers into IIa fibers (also shown by research published by Maffiuletti).