A plot twist is basically an unexpected direction that the narrative takes. They are designed to keep the reader guessing, to maintain a level of interest and atmosphere, and to move the story in a new direction – in other words it helps maintain the momentum of the story and helps to move it forward.
Plot twists are also useful for wrong footing the reader by making them think something might happen in a certain way, when in fact the complete opposite takes place and thus it surprises them. The idea is to make the reader comfortable with the story, and then change direction. This ploy keeps them turning the page.
They are all about creating the unexpected.
The art of a good plot twist needs the full understanding of the story direction and the characters involved, but the twist should always be pertinent to the story. Don’t make the mistake of dropping something into the narrative that has no bearing on the story whatsoever. Plot twists have to be logical; they have to make sense to the reader and they must be connected to the main story.