The discussion here is describing the typical servo-to-dc brushless
motor system commonly used in VCRs.  The speed and phase of the motor
are controlled by a circuit that operates much like the cruise control
on a car.  Usually there are 2 IC's--> 1 that compares a known
reference signal with an incoming feedback signal from the motor and
then develops an error correction signal sent to the motor driver IC.
This speed comparator IC can be either analog or digital processing.
The Motor Drv IC procs the signal into variable amplitude 3-phase ac
power that is frequency commutated by the 3 hall sensors in the motor
stator assy.
To change the motor  speed requires changing the reference input to the
servo control comparator chip.  You can also scale down the feedback
signal from the motor to force the servo to drive it harder, but that
is likely to soften the accuracy and response to motor speed accuracy.
Best to get chip info that will direct you to REF INPUT IC pin.  Try
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