A37575 thank you for your answer. I'm more after the technical background. What happens if you don't stick to the book and simply move the start lever to cutoff (N2 above 46%) and the starter still running?
Technically, the starter valve is electrically controlled and pneumatically operated.
The starter operation is controlled by the start valve.
When selecting the start switch to GRD, a solenoid in the switch is energized and holds the switch in the GRD position until the N2 tachometer indicator switch opens at cutout speed of 46.33% N2. The start switch solenoid is de-energized and the start switch returns to the OFF position.
A sprag clutch disengages the clutch inner race from the clutch outer race after air to the starter is turned off.
In your scenario, when the N2 tachometer switch fails, manually override the solenoid and position the start switch manually to OFF.
As long as the start switch is in the GRD position, and the APU, external source or other engine provides air, the starter will still operate and turn the engine N2 shaft, the engine is "motored".
Once the GRD switch is in the OFF position, air supply to the starter stops and starter re-engagement above 20% N2 can result in starter damage.
Boeing recommends starter re-engagement at 0% N2, 0% - 10% N2 is allowable, 10% - 20% is permissable for purging fuel or to clear a fire but above 20% N2 it is not recommended for any reason.
Refer to your manuals and/or QRH, ask your maintenance for a printout of the AMM 80-11-00 for more details.