The narcissist is a person who has no empathy for others, and in many ways, no real personality of their own. They tend to believe that the world revolves around them, and will not tolerate anything or anyone that does not agree with this view. As a result, both introjection and projection are used as a means for them to cope with their lives and to avoid having to take responsibility for their own actions.
Most narcissists begin relationships with introjection. They take on the positive traits of their target, making their target feel as if the narcissist is a perfect match. They tell them all the right things, do all the right things, and make their target feel like they have truly found their soulmates. This period can last anywhere from a few months to a few years, depending on how much convincing it's going to take for their target to feel like they need the relationship with the narcissist. It's the honeymoon phase of the relationship.
When the honeymoon phase ends in a relationship with a narcissist, however, things do not get comfortable like they do in most relationships. The narcissist needs a constant supply of admiration and attention, and will stop at nothing to get it. They also need to feel that they can do no wrong. This is where projection comes in. They will start to blame everything that goes wrong on the other person. Even if the other person has done nothing at all, they will accuse their target of things that they have done themselves. This can range from small things like not doing the dishes when it was their turn, to much more severe things like domestic violence and abuse.
When it really becomes a problem is when it extends to domestic violence and abuse. They will accuse their victims of abuse, and often wind up sounding even more convincing than the victims themselves. Essentially, they re-victimize the victim, until their victims have nothing left emotionally. This is more common than people would like to believe, and leaves victims of domestic violence feeling as if they have been violated more than once on several levels. Of course, the narcissist is out claiming that they are the victims, and have many people, if not most people convinced. It's because they know what has been done from the inside out, since they are the perpetrators of the crime.
This is something that needs to be stopped if it is caught, but it is hard to catch. When you have two people accusing one another of domestic violence, there is little way of determining who is really telling the truth. Sometimes you just have to trust your instinct, but the sooner these things are caught and stopped, the better off society will be.