The weapon would have been released by a pulse from the NBS if a fully IMC attack was made, the Nav Radar having locked on to his offsets and the system working to specification. There were manual release buttons - one at the front between the pilots for instance - but the system was automated for night/IMC. Numerous checks and authentication procedures had to be complete before any weapon could be released. It was standard procedure to press all the buttons after the NBS release just to ensure the bomb bay was empty before closing the doors and legging it.