"Jane picked up the carving knife" is from no particular point of view.
"Jane picked up the carving knife, her hands trembling" is also from no particular point of view - Jane could notice her hands were trembling and so could an observer.
"Jane picked up the carving knife, her eyes blazing," has to be from an observer's point of view because how could Jane know her eyes were blazing?
"Jane picked up the carving knife, thinking it was now or never" has to be from Jane's point of view, because only Jane can know what she is thinking.
It's really that simple. Where it gets complicated is when people like me ask that scenes are written from a consistent point of view. In other words, once we're in Jane's point of view for a particular scene, please stay in Jane's point of view, and not go into Jack's point of view, or the cat's, or anybody else's who may be hanging around.
1. It can confuse the reader. For example...
Miranda washed up the dishes, thinking Eleanor was incredibly lazy for not helping. Eleanor examined her fingers, admiring the expensive manicure she'd had done only yesterday. Billy was bound to fall in love with her now. Miranda slapped down the last tea cup. What a cow.
"Have you been watching Strictly Come Dancing?" she said.
Who is speaking? It could be either Eleanor or Miranda.
2. If you go backwards and forwards from one character to another, even if the reader can follow, it can feel like being at a tennis match and watching the ball going between the two players. At worst, you can feel almost seasick.
3. Even if the reader can follow easily, it takes them away from what is happening in the scene and a bit of their brain is distracted into working out something technical which should have been hidden. If their brain gets too distracted they'll put the book down and go and do something else.
4. Staying in one character's viewpoint means we feel we're in the head of that character, and if we're in their head, we're engaged with the character, the story and the writing. Which is what you want, isn't it?
5. I know that lots of best selling writers do change point of view within a scene but if you noticed then it means you came out of the story for a moment. Are you certain your story telling is so good that you can afford for a reader to come out of the writing and work out who is speaking now?
6. It's such an easy thing to spot. On a first page it shows carelessness at best, ignorance at worst. An agent or publisher may not bother to read on simply because of shifting POV. In class I have to admit it's wonderfully easy to give POV comments, and not strain the brain into coming up with something else, As a tutor, if I spend time commenting on POV - which is important for the above 5 reasons - I may not have time to get round to other comments, like getting all the easy jobs done on the To Do list and putting off the tricky stuff.
Just be consistent. If you start a scene in Jane's POV, stick with it, don't get sidetracked into Jack's (or Jill's). Write the next scene from Jack's POV if you want, and the scene after that from Jill's, but don't mix them up within a scene. Keep us in the POV character's head, and keep us reading.