He tricked the wooden Cyberman, created to be too primitive to set off the alarms, into destroying itself after using the sonic and the truth field to make it think its gun had been turned around. [quote]And, for that matter, how was he able to tell the boy who wasn't actually Barnable that he had a plan if he didn't?[/quote] Because why would he tell him that he plans to go up to the Daleks and die? He's giving the people he's protected for centuries one last bit of hope before he pops off.