Each relay is switching the polarity going to the motor wire, one relay on each motor wire. So with no relays triggered both motor wires would be at ground = no voltage across motor = no movement. If one relay is triggered, that motor wire goes to the +voltage coming from the track power adaptor. This causes the motor to move in one direction. Vice versa for the other relay being triggered, the motor will move in the opposite direction.
If both relays are triggered, both motor wires will be at the same +voltage, meaning no voltage difference across the motor which means no current will flow.. no movement.
By using two relays you get dynamic braking, once you release both relays the motor wires will be held to ground and the motor will resist turning (back EMF).
If you use the other method-- one DPDT relay (for direction) and one SPST (for enable), then when you 'disable' using the SPST the motor wire(s) will float and the motor will drift to a stop, not stop rapidly as with dynamic braking.
If I've missed something, or it doesn't look right.. let me know.