One of the so-called rules that I often hear about from unpublished writers is the sin of head hopping. Often they do not understand what head hopping is, and apply it to any switch in point of view during a scene.
True head hopping is jumping from character to character without telling the reader whose head you are in, and without regard to the build up of reader identification with the character. Remember that the reader is always searching for a character/hero to identify with. They want that connection.
Let me say that truly powerful story tellers can and do switch points of views in the middle of scenes. Some like Larry McMurtry or Peter Ackeroyd or Terry Pratchett switich quite often. Nora Roberts is another author who springs to mind. But they are all superlative story tellers. Ultimately, the story telling talent will carry the reader along.
Right, why then do people go about staying in a specific point of view? Why is there all this fuss?
The reason is reader identification with the character, in other words -- connecting with the reader.
If the reader is going to be inside a character's head, the reader wants to know which head she is in. It is disconcerting for the reader to be pulled out of the story because she thought she was in the heroine's head and it turns out that the writer has dipped into the housekeeper's head for a moment.
Or another way to look at it is that the reader is always seeing a scene through a filter, whether it is the filter of a character or even the filter of the ominscient narrator. Without that filter, the reader has no idea how to interpret the scene. If filters are changed awkwardly, or the reader thinks she is seeing through a specific filter and finds out differently, the reader may get pulled out of the scene.
Thus it is the awkwardness of the shifts without sufficent tension/page turning quality that causes headhopping. If there is sufficient page turning quality, the vast majority of readers will forgive an awkward point of view shift. It is really ALL IN THE EXECUTION rather than in some hard and fast rule that says each chapter must be only shown from one character's point of view.
One way of thinking about it is that each time you shift POV, you are starting a new scene, so that scene needs to be anchored. And as scenes need to be in general more than one paragraph long, once you go into a character's head expect to stay there for awhile. It is not a one sentence deal.
I used to show the shifts in point of view as scenes breaks, but my editors did away with the breaks. However, I still think of the shifts between hero and heroine as changes in scenes. It helps to keep me focused. Do I really need to a scene change or can I show the other character's thoughts through some telling detail?
Shifting from one deep point of view to another can be disconcerting, so if you are going to shift, shift at a natural break. Think camera shots. Sometimes I will shift point of views after a bit of dialogue which does not have speech attributes.
I find by using shifts in POV, it means that I do not have to do sequels or reprises of scenes as sometimes both reactions of the characters are important. A reprise can decrease tension far more quickly than a well-timed, well executed shift in point of view. It is all about maintaining tension.
Switching Point of View is something that comes with practice. In order to be able to switch, you first have to be hold a Point of view. Some people find it useful to write in different colours for different points of view -- say pink for the heroine and blue for the hero. I find it easier simply to think -- whose Point of View am I in? Through whose eyes am I seeing the scene? And how does the reader know whose head? What words would that character use that are unique to that character? And when/why do I need to shift Point of View?
So hopefully now, the rationale behind switching Point of View becomes clear.
There is NO hard and fast rule. The only rule is the story. If the story flows and the tension is high, you can shift as the story dictates. If the tension is low, not even slavish devotion to one point of view will save it.
Techniques are there to be mastered, rather than followed blindly.