Pluto having the 'upper hand' in relationships, what does it actually mean?

I advise people to stay out of heavy pluto type relationships because they generally cause a lot of unneccessary pain. 

First thing to look for in synastry:

An imbalance in pluto contacts

If you are online looking up someone's natal chart, there is a good chance you are probably already slightly obsessed with this person. The first thing to look for is an imbalance in pluto contacts. What does this mean? If the persons pluto contacts any of your personal planets sun, moon, mars, venus in particular, and your pluto does not contact any of theirs, then they, simply put, will effect you. They have power over you. There is no way you can win except by recognizing this and walking away at the start. Pick your battles. They may or may not try to exercise this power depending on what kind of person they are (this can be seen from their natal chart). So how does this play out?

Their pluto, your sun (conjunction, square or opposition)
Pluto is everything subconcious and hidden and...dark. Think of what the sun represents. Light, illumination and clarity.  Your presence is going to illuminate their dark side. Whatever their dark side may be. This doesn't necessarily have to be obsession, although for a lot of people it is. It can also be anything negative that they have been trying to keep under concious control.
For example, if they have alcoholic tendencies and are struggling with this, having you around will make it worse. They will tend to drink more.
If they were raised in a controlling way, and have been repressing negative childhood memories, this will come up. They might become controlling toward you to reinact the negativity and attempt to purge themselves of it.

The thing is, they will be acting at a suboncious level and can therefore walk away when are done purging, transformed, while you, the sun, are also transformed in that you feel like you have just witnessed something dirty and negative. Something that you cannot "unsee". It's like walking in on your parents having sex when you are 10 years old. That's Pluto, you're innocent. Did you really have to witness that? Nope. Was it neccessary for your spiritual growth? Nope. Pluto doesn't care. It just transforms you, most times for the worse. Not a good thing.
Also keep in mind that pluto is not a personal planet. Therefore the pluto person will feel nothing after vomiting all the negativity in your face. Run away!!!

Their pluto, your moon. (Conjunction, square, opposition)
This is a bad one because your feelings are involved. In many cases it will seem like the Pluto person is trying to manipulate your feelings in any way they can. That is how you, as the moon will experience this aspect. They could be the nicest person in the world to everyone else. You might also be flattered by the obsession. You might be flattered at the "crazy" way  in which they act when they're around you. Because people only act crazy when they are in love right? They must feel for you too. Guess again. They only thing pluto is doing is acting like a bipolar schizophrenic with tourettes syndrome. Does someone like that know what they are doing? No. Run away even faster!

 More later!