Where can I find........

In attempting to help someone else dealing with a similar situation, I am trying to find a good writing or explanation of why an NPD will break things off with a person, but then keep them dangling on the vine, toying with them like a cat does with its prey. Hoovering, etc. ? What is the clinical reason they do this? I have read a lot here on this site and may have overlooked it. Does Sam Vaknin have anything explaining it? Thanks in advance. If you could just point me in the right direction.