I believe the theory behind not leaning is to subcommunicate that you are not needy. Thus you become higher value. Plus if she leans in she is working for you, which again = higher value. And some 'gurus' think the harder she works and the more she invests in the interaction (once hooked), the more attracted she'll be.

However, like everything I'm sure it's effectiveness will be situational.

But there's probably no 'one true way' to this stuff. What works for one might not work for another. And there's never going to be any empirical proof that it works anyway.