Simon,
There are two ways of implementing the spindle direction, and it depends on the speed controller you are using:
1. If you have a VFD, you could use the electromechanical relay to change between the CW and CCW of the VFD. You would activate this with the spindle direction pin.
2. If you had a C23, C11, or C6, which come with two independent relays; one could be used for CW and the other one for CCW. You would activate this with by assigning the two relays under config/Ports&Pins/Spindle/ Relays.
3. If your speed controller did not have a direction input, and was DC based, you could use the two relays to reverse the winding of the motor. You have to be careful to allow for a delay before changing directions and watching not to exceed the rated amperage of the relays.
Arturo Duncan
http://cnc4pc.com