I know some writers who say that when the ideas are coming they come so hard and fast they can't type quick enough. I have those experiences at times, and I'm very grateful for them when they come - except for those 2 AM diatribes. My muse could stand some Lunesta.
But my experience when I'm actually writing, or even just daydreaming (I call that plotting) is that my brain actually feels s-l-o-w-e-d d-o-w-n. I'm sure that my eyes glaze over and a little white fade-out effect takes over whatever room I happen to be in at the time, as well.
There's a literal feeling of settling in a corner of my brain, like the WIP's ass just found an awesome chair with a perfect-fit buttmark and tossed itself down. It gets settled, curls up (don't quote me on this, but it might be purring) and that's when the real act of writing starts happening.
It's typically about two pages in to that day's work, those two pages will need a lot of assistance and a Crap Removal Team when the time comes. I feel like the idea is walking around my head, testing different areas while I toss out the requisite two pages of drivel, trying out chairs and dismissing them.
But once it finds that buttmark chair... we're in business.