Kirks G - 'going past TOD' takes us well off topic, but unless a later update has changed things, 'Descend Now' is only available before VNAV TOD. Cruise Speed will begin to reduce towards min clean (to attempt to control total energy) until descent is 'authorised', whereupon Starship Enterprise will dive as fast as it can (unfortunately into V/Mmo land) until either you do something else or 'Path' is regained. Your last sentence is correct.