1) The cockpit door keycode is used to call the flight deck to open the door. At that point a pilot would verify the person trying to access the cockpit and only then swictch a switch to "release" the magnetic locks securing the door closed.
2) If the "in cockpit" pilot is unconcious while the other is out of the cockpit, a different code can be entered from the outside with a timer delay which would auto open the door if not permanently locked (using the deadbolt) by the pilot inside the cockpit. (This timer delay is there to allow a number of seconds to pass with the buzzer sounding in the cockpit for the "incapacitated" pilot to open the door using the console switch. Should the incapacitated pilot not open the door then the normal (magnetised) door locks would automatically open after the timer has expired.
3) Should any of the codes be used to try to gain access to the cockpit then, a lock door (spring loaded) switch can be held in the cockpit by a pilot until the "deadbolt" is put on to prevent the entry into the cockpit. To put the deadbolt on and prevent permanent access to the cockpit, a pilot has to get out of his seat normally and "slide/move" the deadbolt across the door preventing any access if the door was tried to be opened.
If the deadbolt was pushed across the door from the inside of the cockpit then, no matter what normal actions someone takes from outside the door, you will not be able to gain access.