The control driver is supplied with nominally +5V used for the driver circuitry and for the motor supply.
A voltage regulator can set the motor supply to anywhere between the +5V and 0V.
Maximum rotating speed and torque is achieved at +5V. Lowering the motor supply will lower the rotating speed (and torque). How low the motor supply can be set before the torque is unable to win over the static friction, and therefore not turn, depends on the actual application, but typically occurs at around 1V.
Lowering motor supply during ‘start & stop’ can make a ‘start/stop sweep softer.