How is a higher torque achieved in a single phase cap-start cap-run motor?
This is not completely true. A cap motor can use 2 capacitors. One big for the starting torque with more amps and one smaller for driving and to create the 90% phase shift for the 2nd winding to turn. The switch you mentioned doesn't cut the starting winding but only the starting capacitor. To achieve a better starting torque as said you need bigger capacitance thus 2caps are connected in parallel. After the motor has achieved enough n it cuts one cap from the circuit and leaves the motor with a smaller capacitor. Having a big capacitor while the motor is running can create high currents and burn the motor. And the starting winding can't be cut because you need 2 windings with 90° current phase shift to make it turn.