How I focus 100% on what I'm interested in, so earlier I opened a Care Package; someone sent me an old iPad, some tea, an old Sony Reader, and I got down to figuring out how to hook up the iPad, played around with it--never had one before; heard of but never saw one either. And all of a sudden an hour or more later I noticed my hands were freezing, my back hurt a lot. I had turned off the oil heaters, forgot about spine disease, and was having fun.
When I do what I enjoy, my body is on a back burner, mentally--if it's mapped at all, and I am all mental energy.
I call it "zoning," but that's misleading. Zoning is a kind of day-dreaming, or perhaps distracted, "not here," whereas focusing is being all here, with nothing left for anything else, including--for me, noticing my body's needs.