Unlike ordinary DC motors, which spin freely when power is applied, steppers require that their power source be continuously pulsed in specific patterns. These patterns, or step sequences, determine the speed and direction of a stepper's motion. The fixed stepping angle gives steppers their precision. As long as the motor's maximum limits of speed or torque are not exceeded, the controlling prog ram knows a stepper's precise position at any given time. This pushes and pulls at permanent magnets arranged around the edge of a rotor that drives the output shaft.
Bipolar steppers require that the polarity of power to the coils be reversed.
After all, it's the sequence for which all of the manufacturer's specifications for torque, step angle, and speed apply. The motor's speed depends on how fast the controller runs through the step sequence. At any time the controller can stop in mid sequence. If it leaves power to any pair of energized coils on, the motor is locked in place by their magnetic fields.