The description of the operation is slightly wrong.
The diode across the motor isn't just to spike-suppress. The system is actually a switching regulator trading voltage for current. The motor doubles as the inductor and the "protective" diode is actually the free-wheeling diode.
With the motor stalled the current through it, and thus the torque it exerts, climbs until limited by the resistance of the winding, wiring, and voltage drop of the freewheeling diode. (If it were a superconductor and the diode ideal it would climb forever - or until the motor started.)