When you move on from the narcissist, when you leave and you cut contact with them, when you move on with your life... they feel as though they no longer have the ability to control you. They feel as though they no longer have the power to influence or direct your behavior or the cause of events. They only had the ability to do this because they tricked you into giving your power to them. They have no power of their own. They have to manipulate you into handing your power over to them, so that they can then possess the ability to control you, to maintain influence and authority over you.
When you leave and cut contact with them, when you move on with your life, they no longer have the ability to control you, because you are no longer giving your power to them. You cannot move on without holding your own power and using it to make that decision to move on from them. Which then leaves them powerless, it leaves them feeling helpless without ability, influence or power. It also leaves them unable to control themselves. But this isn't because you have taken something from them, it is because you have stopped given them something that rightfully belongs to you.
You have stopped given them your ability, influence and power. Instead of handing it over to them where they can then use it to control you, you have made the decision to keep it to yourself. Which then leaves the narcissist feeling helpless. Without a constant source of supply to depend on, the narcissist has no ability, influence or power of their own. They are out of control within their minds, they have difficulty controlling themselves. But what keeps their behavior in check is when they can use you as their emotional regulation tool. They use you to regulate their emotions to maintain their false sense of self worth, self esteem and self-importance. Which is why they always had such a desperate need to control you. Your freedom posed a great threat to how they saw themselves.
The narcissist is imprisoned in their own minds, but for them to get a taste of what it really feels like to be free they have to imprison and enslave their victims. It creates the illusion that they are free when they observe the boundaries that they have created for you. When they lock you into their world it creates the illusion of freedom for them. But they are not free, they are actually being controlled by their own minds and emotions. The only way that they can regain any control or even the illusion of control over their own minds and their emotions is to control you. They have to project their flaws and insecurities onto you, they have to try to make you feel the way that they do. Only then can they feel better about themselves, only then can they feel like they are free. But even then it isn't real it's just an illusion of freedom.
When you leave the narcissist, when you cut contact with them, that is when they begin to realise that they never had any ability, influence or power. They begin to realise that it was yours all along. They begin to realise that they are not free and they never were, they just created the illusion that they were free by imprisoning or enslaving you.
When you finally leave the narcissist, that is when they become imprisoned and enslaved by a prison of their own making.