Deep Hack Mode is of of those curious things about loving what you do. It's the action of being so engrossed in something that your rational mind and the outside world no longer exist -- like in computer hacking, in regular hack mode, where you are communicating with the machine on it's own level, in DHM, you no longer have to mentally think about communication with it; it just sort of happens.
It could be said it's kind of like meditation state, only more action based. In me, what it amounts to is I get so focused that I loose awareness of the outside world, and even my own self to the point I've been known to not blink for minutes at a time. Your concious mind almost detaches from the rest.
Another good example is that, under normal circustances, I cannot touch type. I'm a heavy-grade coder, yes, but I still glance down to my keyboard once every few seconds or so. After I've been coding for a few hours and have reached that blissful state of Hack Mode, I am finally able to touch type, and in Deep Hack Mode I don't even think about my fingers or even care how my thoughts are getting into the machine. It's as though all of these complex functions are handed off to other parts of the mind and you no longer have to consider them.
The upside of a session of Hack Mode is I can write a log of high-quality code and I don't seem (to me) to take much time. The downside is I tend to loose any reference on the outside world, most important of this is time -- you can loose hours at a time because you loose focus of anything outside of the text on the display.