Just to add to what has been said. The starter is engaged to the N2 gearbox via a ratchet pawl Assy on most aircraft.
Think of the starter in two parts, the connection into the gearbox once N2 speed is high enough disengages the sprung pawls via centrifugal force disconnecting the turbine/planetary gear section.
When this section is disconnected and you send pneumatics to the turbine it spins up blowing it to pieces. Starters can be crash engaged at certain speeds, but this can damage the starter, post checks are actioned.
QRH re-start envelope N2 speeds are pretty much spot on.