Rory wrote a post entitled "Whole Brain Coding" a couple of days ago, in which he asserts that coding requires both the left and right halves of the brain, the left brain working on the sequential and analytical parts of the task, and the right brain working on the intuitive and holistic parts (reverse these if you live in the southern hemisphere...)
When things are going well and you're in the "flow", my guess is that you're seeing involvement of both sides of the brain, but I'm not sure that that's all there is to it (I'm not asserting that Rory said that). I did a few searches to try to see what research had been done into the "flow", but didn't come up with much. There is:
In the Zone: A Bio-Behavioristic Analysis of Csikszentmihalyi's Flow Experience
but I have a hard time parsing sentences like:
Primarily, the decision making process behind such behaviors as disparate as creative thinking, problem solving, or walking to the store are all dependent upon and influenced by somatic or neural activation variables that are mediated by abstract environmental contingencies.
I think that's saying, "The way we make decisions is dependent on what's going on around us", which makes me happy that I'm not a psychologist who has to read and write papers like that.
Understanding the Psychology of Programming
which is a light intro to the topic.
On the whole math vs. coding thing, though I have a math minor and enjoyed my math classes up through linear algebra and multivariable classes, I ended up in software for two reasons: