nonreactiveness indicates being unmoved, still, calm - these are signs of strength, that's why women are attracted to it.
simple example, if you're out one day with your woman and there's an explosion in the distance; do you think she'd be more attracted to you if you jumped and started screaming, or if you merely looked over and were nonreactive?
not only that but being nonreactive indicates you're not impressed or overly interested. shows that you have options, shows that you don't get attached easily, and shows that you see yourself as being above the woman - the last one in particular makes her think you are above her.
in contrast to the guy that shows extreme interest and involvement with the woman, that shows that she is above him. future divorce/cheating relationship right there.
