In any relationship, the person with the most power is the one who needs the other the least.
Control in a healthy relationship passes back and forth as desire and need dictate for each partner. In an unhealthy relationship you have an unbalanced manipulation of this control by a partner.
Although control is never in complete balance, it becomes manipulation when one partner, in essence blackmails, the other with what would otherwise be a reinforce for the manipulated under a healthy circumstance.
This happens for a plethora different reasons, but the condition comes about by two ways – the submissive participant becomes conditioned to allow the manipulation to occur and/or the dominate initiates the manipulation.
In either case the rule still holds true – the one who needs the other the least has the most control. Nowhere is this more evident than in interpersonal relationships.
'The Rational Male' by Rollo Tomassi
https://www.amazon.com/dp/B00FK901R8