It's a
paradox of
S and M that it is actually the submissive, not the
dominant, that is truly in
control of the
sexual encounter.
This may sound strange, but it's true. In a proper S and M relationship, it is the job of the dominant to produce a range of sensations in the submissive; the submissive is there to enjoy them. It is the submissive who can end the encounter, by letting the dominant know when enough is enough.
There are some dominants who don't follow this convention, but they tend to have a hard time finding partners...
By the way... S and M is short for sadism and masochism; B and D is short for bondage and domination. Some people say that they're two separate (but related) fetishes, others say that S and M is just the extreme end of B and D.