[Radiosmurf]

The 4000L and IC both used the same motor.
At rest three contacts attached to the motor shaft rest against a commutator type arrangement of fixed contacts which are connected to the the other end of the motor from the drive coil connections. This provides the necessary conditions for the motor to start rotating. I believe that poor/dirty commutator contact is the cause of the reluctant starting.
Centrifugal force (OK, I know there's no such thing but you know what I mean!) acting on a small weight attached to each of the three shaft contacts within the motor causes each contact to be lifted from the commutator as rotation begins. The initial rotation is maintained by the back EMF from each drive coil, via the three diodes, switching each of the three pairs of drive transistors in turn thus setting up a three phase drive arrangement.
Speed control is achieved by this back EMF being summed and compared to a voltage set by the Zener diode and associated speed adjustment pot.
Some motor control boards also have two pots which are used to balance two of the drive currents against the other fixed drive transistor current.
The Monitor series of machines used a similar but different motor and to my knowledge never suffered the same fault.