The spindle speed can be easily specified and the spindle can be turned on in a forward or reverse direction. It can also, of course, be turned off.
An "S" word is used to specify the spindle speed (in RPM for machining centres). An M03 is used to turn the spindle on in a clockwise (CW, forward) manner. M04 turns the spindle on in a counter clockwise (CCW) manner.
M05 turns the spindle off. Note that turning centres also have a feature called constant surface speed which allows spindle speed to also be specified in surface feet per minute (or meters per minute).
It calculates spindle speed so the surface speed is kept as specified with a tool position change. It then supplies a voltage, corresponding to the calculated spindle speed, to the spindle control to rotate the spindle at the correct surface speed when the surface speed is specified after S.