The starter motor drives through a clutch, could be a 'sprag clutch' or a ratchet/pawl assembly. With the starter off the pawls just ride over the ratchet, as the N2 turns the gearbox, engaging the starter, as it starts to spin up the pawls engage and drive the ratchet gear and gearbox. The problems that could arise if the engine is turning the starter could overspeed before it engages (due to no load) damaging the starter and/or gearbox. Thats why on a failed start always wait until the N2 is zero before re-engaging. However if there should be a tail pipe fire or an attempt be required to restart a flamed out engine for instance that precaution could be ignored but it would probably (depending on aircraft/engine type) require to be written up as there may be additional checks required.
