Re: How do you train/achieve nothingness?
I'd say (from personal experience) that in combat, there are two separate - but connected - factors involved: a state of hyper-readiness and aggression (a controlled form of "fight-or-flight" instinct), and complete in-the-moment focus.
The former provides adrenaline, which causes the perception that time and movement have slowed down, and that objects in your immediate area seem larger and more looming.
The latter is the state of "flow" or "groove" we discussed, which puts you in utter focus and concentration on the task at hand. In tandem, these factors permit the coordinated actions of combat (or simulated combat).
It's definitely not a state of being zoned out or just being focused on one particular thing. I believe it comes from having trained in the "whole package." A violinist may become lost in the moment of his music-making, to the exclusion of all that is around him (There are many accounts of musicians saying that they were not aware of the audience being there, or even of the walls of the concert hall), but a martial artist can't afford to be. He/she must have a spatial awareness more akin to that of a trapeze artist or other performer whose surroundings (and other individuals) play a key role in his activity and his survival.