I am in a D/s relationship (Part of BDSM)
Neither me or my Gf enjoy the pain side of things, so we don't go into that side of it.
Just to dispel the common misconceptions about the relationships for people who don't understand them: (I'm aware most here are probably fine with it and don't think it wrong, but I always hope that anyone who does, who reads my post will understand it better.)
1. Being part of BDSM is not a mental problem, it is a lifestyle choice that people enjoy and does NOT mean there is anything wrong with them.
2. A very important point is everything done in BDSM should be safe, sane and consensual. Anyone involved must be aware and agree to what they want to happen and must be safe about how they go about it.
3. It is not only about pain, the letters stand for (Bondage and Discipline) (Domination and Submission) (Sadism) and (Masochism), while some may enjoy pain, this is only part of BDSM and is not done by all, from most I know, those that do enjoy pleasure through pain is actually the minority of the group.
And as others have said, it's not immediately apparent, but the sub really has all the power, they choose who to give up control too, and can choose to take it back.