Theoretical Zen is the use of koans and riddles to bring about epiphany or enlightenment, the end result being a 'Zen' of whatever, for example the Zen of meditation is easy, the Zen of daily life is a bit more difficult. Ultimately, it is the understanding that all is oneness.
Practical Zen is the us of such knowledge in daily life - not just knowing, but being. Regardless of what comes at you.
I've been using the practical aspect lately, as a lot of things are trying to goad me to break my inner balance and I'm not just simply not letting it; I'm transcending everything the unbalancing effect would use to disrupt it.
The interesting thing is that what results in practicing this mentality is a several-days-long period of inner turmoil, which if it never escapes into the physical, eventually transmutes itself and a Stone is had in the process.
Example: Last night at work was awful. I was sore for no reason, tired beyond description (with a full night's sleep) and bad juju from the day before being unimaginably nightmarish and no set goals were accomplished. All night long, I pushed inward, not letting it affect me or those around me - the only indication was someone saying I looked tired (I think it was the dead eyes they referred to) but the end result was a very lucrative night of tips, having been sure all night that my inner sourness would become manifest as a crappy night.
The real theoretical zen in all of this is the fact that I'm using this method to accumulate money, meanwhile being as enlightened as I am. To use the noble path is to not think of it or see that path anywhere.