- It will show him that you are a woman of high value. It will demonstrate that you are a woman who isn't desperate or needy - a woman who can live without him (because you can). Men find this attractive in a woman. Although this demonstration is unlikely to increase his attraction for you enough to make him change his decision, you will avoid confirming him in his decision by showing him that you are needy, pathetic and desperate - all of which are obvious symptoms of being below his league.
- It will show him what he is missing by way of contrast. By forcing him to experience life with you, and then suddenly without you (in other words, before and after the break up), you will emphasize whatever feelings he has as a result of his decision. First he has you, then he doesn't. The starker that contrast is, the more he will feel your absence, and the more likely it is that he will miss you and want you back. Again, it is unlikely that this effect will be enough to make him change his mind, but in some instances it will be. (Note: it is also possible that cutting him off will show him how much he doesn't miss you, but if this is this case then there is nothing you can gain by contacting him anyway - it is unquestionably over.)
- It will force him to think about his options seriously. As long you stay in touch with him, he will know that he can have you back at any moment, and he will feel no urgency to reverse his decision. By staying in touch with him, you effectively give him a "safety net." Even if he doubts his decision, he will have nothing forcing him to act one way or the other. He will simply go about his life, waiting until his “feelings about you become clear” (which they never will).
- It will empower you. Although you have no control over his decision to break up with you or stop dating you, you can control what happens as a result of that decision. In fact, you are the sole person capable of "enforcing" the consequences of his decision. By hanging on and hoping to get him back, you give him all of the power. But if you force him to live with the consequences of his decision (i.e. making him live without you when he says "it's over"), you exercise your agency, your power. You seize what little control you can in the interaction. Although this probably won't change the outcome of the situation, it will go a long way towards bolstering your ego and self-esteem in the face of his rejection. You will transform yourself from someone who is pathetic and dejected in light of inevitable circumstances, into a person who exercises influence on the outcome of her situation - and her ex's situation. The added benefit (as explained in the other points of this post) is that by doing so you don't even sacrifice any of the likelihood that he will take you back - in fact, if anything, you increase it.
- It will make him doubt his future options. This is probably the most important mechanism at work in your attempt to get him back, and it will be powerful in proportion to the length of the relationship with you that he is ending. If you stay in touch with him in an attempt to salvage things, you will confirm his belief that he can find someone better (which is his implicit motive for dumping you). The longer you stay in touch with him after the break up, the more he will think "Hmm... if she wants me so badly, girls who are hotter and sweeter will probably like me enough to date me too. Why would I take her back when I know I could do better?” While he might not be so calculating as to have this thought explicitly, I absolutely guarantee you that this thought passes through his subconscious and has a profound influence on his thinking about the break up. And it will occur to him even if you aren't making an active effort to get him back, but "just" texting or calling each other from time to time. He knows you wouldn't stay in touch unless you were interested in getting back together - he knows being "just friends" isn't workable. But, if you cut him off, he won’t know whether or not he just threw away the best girl he could get, and that doubt is priceless in making him consider coming back to you.
If you liked this post, you'll definitely like my book, Beyond the Breakup, in which I add a 6th point that I overlooked when writing this post originally. I also explain a lot more about the practical side of cutting a guy off - what to say, when is "too late," and what to do if he gets back in touch.