Head-hopping is when a writer expresses multiple characters' thoughts or points of views (POV) within the same scene. If a scene starts with our heroine, Jane Doe, watching the hero, John Smith, walk into the room and the writer starts describing Jane's facial expressions, then the reader knows it's no longer Jane's thoughts being expressed. Because, who thinks of his or her own facial expressions while thinking? So who's watching Jane's face and wondering what she's thinking? Is it John? Maybe. Maybe not. What if John's evil twin brother Josh is in the room?
Not establishing a clear POV can leave the reader scratching his or her head and wondering. Is John watching Jane's face light up, hoping he put that smile on her face? Or is Josh watching Jane, plotting ways to foil her budding relationship with his brother?
When an author jumps from one character's thoughts and POV willie-nillie within the same scene, it can be confusing. It also makes it difficlt for the reader to connect with the characters. And suddenly switching viewpoints in the middle of a scene, or worse, in the middle of a character dialogue, can not only leave the reader scratching his/her head, it can throw the reader completely out of the story.
If a character speaks or thinks or notices another character, the reader needs to know which character is doing the speaking, thinking, and notcing. It's usually easy to tell which character is speaking because the author will use action or dialogue tags. But during segments of internal dialogue or thought, the author needs to establish POV early on so the reader can connect to that charater.
This is not to say that a writer can't write in multiple POV. But switching POV whenever a character speaks will drive a reader crazy. In general, most readers don't care what a minor, secondary character is thinking. If it adds nothing to the plot or storyline, then don't add it at all. Stick to the main character's POV and only switch at scene breaks or chapter breaks. Occassionally, a writer can change POV flawlessly within the same scene without confusing the reader. But it takes skill and finess.
One very famous romance writer head hops, but for the most part, she has that skill and finess required to keep the reader imersed in the story without the confussion often caused by head-hopping.
A multi-published author in my local chapter compared successful POV changes within a scene to a video camera. When a camera is focused on one subject we see that subject. The same with the written scene. When in the heroine's POV, the reader is focused on the heroine. In order for the camera to focus on another subject, it must first zoom out. If the person behind the camera tries to pan over to another subject while still zoomed in, the film will blur and those watching will get dizzy. So, the camera must first zoom out, pan the room, and then zoom in on the next subject.
It's the same with writing. In order to change POV, the writer must first zoom out, pan, and then zoom onto the next character before expressing his/her POV. And that's what I try to do.
If I'm going to change POV without using scene breaks, then I pull back from the character whose POV I'm currently in, insert a descriptive statement or two and an action or dialogue tag and then slip into the next character's POV. And I stay in that character's POV until the next scene or chapter.
This technique seems to work well if not abused and it doesn't feel as if the author is using an omnipotent POV to accomplish the switch. It also enables the author to slip into a deep POV so the reader can "feel" what another character feels within the scene without confusing the reader.
In my historical, Slightly Tarnished, released yesterday, I used this technique of panning back and zooming in to switch POV's, and I think it works. Check it out and see for yourself. http://www.wildrosepress.us/maincatalog/index.php?main_page=product_info&cPath=191&products_id=4543 Slightly Tarnished is an historical, but there is an element of the paranormal at the end. I just couldn't help myself.