The 680 downlocks, like all citations, are integral to the actuators. In simple terms there is a key way in the piston which when in the down position a pair of integral hooks fall into thus locking the actuator and gear down. A 'collar' then keeps them there. The trapped hyd pressure then ensures the collar does not move. The only way to unlock is with retract hyd pressure.
Any time the actuator travels to the down lock position it will lock, however it is not positive enough to bare the weight of the aircraft. Therefore, although the gear will free fall to down and locked and give three greens, there is a nitrogen bottle which can be discharged, applying the pressure required to positively lock.
Hope this helps