The world rushes in
That's the feeling you get when you're present.
When you're stuck in your head, thought stands between you and the world. Like an obstacle, or a veil over your eyes: the outside world is blocked out, or heavily filtered.
When you become present again, the veil lifts.
The world reappears in all its color, all its presence. You don't need to pay attention to things: they're there, alive, vibrant, inescapable.
With the unshakable certainty that you're part of it.