We still speak of writing in the old way, although the process is different for most of us these days. Saying, "The keyboard is mightier than the sword" just isn't as poetic. Write refers to composing, arranging words in order, whether we use an old-fashioned, #2 pencil or the latest toy from the wonderful world of technology.
The hand of the writer, then, is metaphorical. It operates in the background, creating a world for readers to dive into and remain immersed in for a while.
Nothing makes me happier than when a fan says, "I lost all track of time," or "I couldn't put it down," in reference to my work. If I'm doing my job, I cast a mini-spell over a reader, and she's pulled into the world I create. She shouldn't want to leave, at least until I'm done with her.
There are times, however, when the hand of the writer is visible in a book, when a reader feels it pushing her in a direction or pulling the narrative down a path that doesn't feel natural. At that point the spell breaks, and the reader is aware she is being told a story. If the story is good enough, that might not matter a great deal, but if the writer's hand is seen pushing and pulling a second time or even a third, the reader begins to feel that things aren't quite right.
Sometimes it's a plot device that isn't natural. It might be a character who acts in ways contrary to what we know of him. It could be a detour from the storyline so the author can tell us what she knows about popcorn production or slavery in ancient Sumeria. Anything that niggles at the reader's bubble of concentration can pop it, and she looks around and thinks, "Wait. All this isn't real. I can see the writer's hand between the lines."
At that point, she might put the book down and go make supper. Good for her family, maybe. Not so good for the author who allowed that hand--or keyboard, or whatever--to peep through the magic.