For motors with multiple poles the electrical rotation does not correspond to a mechanical rotation.
A four pole BLDC motor uses four electrical rotation cycles to have one mechanical
rotation.
The strength of the magnetic field determines the force and speed of the motor. By varying the
current flow through the coils, the speed and torque of the motor can be adjusted. The most
common way to control the current flow is to control the average current flow through the coils.
PWM (Pulse Width Modulation) is used to adjust the average voltage and thereby the average
current, inducing the speed. For example, the PWM frequency selected is the range from 10kHz
to 200kHz according to the application (commutation losses, audible frequency...).