False colours

Usually I use Photoshop for only the boring, mundane things like resizing pix or cropping or taking out that blob that my camera adds. And I know the keystroke for getting up Curves, and what to do to make the pic lighter. The other day the cursor slipped and this suddenly appeared -- It's a bit dark, but who knew there was the potential for a dark red glow in it?

Take the curve the other way, and you can get this -

Magic. I don't understand how/why it ends up like that. I don't particularly want to understand, or to do a lot of this sort of image manipulation -- it's a bit like having an ice cream when you're on holiday, not something for the daily diet!