From my understanding of these units, this explanation raises a question about certain departures.
For instance, say the SID called for runway heading, followed by vectors to a fix. I was told that the way to do it is the plane departs and uses the HDG button until you are ready to engage the flightplan you programmed into FMS. At that point, you'd use the NAV button to tell the autopilot to follow the RNAV route.
If that's not true, how would you mimic that type of departure?
You can do it that way. when the HDG mode is on press the LNAV button, LNAV will activate.
You can press it before or after, but LNAV will not activate unless the HDG mode is on.
Keep in mind LNAV will not fly an intercept course, when turned on, it turns the aircraft directly to the next flight plan waypoint. On reaching that waypoint it will then fly the flight plan track.
Whereas in some real Airliners LNAV won't even active unless you're currently on an intercept course (you'd get an error message from the FMS indicating this).