Lawrence,
what you get on the ND is a dotted-white line starting from the apex of the triangle with a projected arc towards the direct-to waypoint (if it's not straight ahead) as opposed to the active route which is magenta in color. if you do not execute the modified route, the dotted-white line (modified) will be continuously be updated until you execute the route. the new track towards the new waypoint is continously updated on the LEGS page and you can even use HDG SEL to manually turn the airplane towards the new waypoint before executing the FMC to ensure a 'smoother' turn. hope i'm not confusing you further...cheers and merry xmas.
SR