BDSM is basically the utilization of the physical sexual charge to "power" particular psychological / behavioral constructs. The sexual premise along with the users' control faculties works as a regulator of whichever constructs are being used. Take away this regulating function and things like dominance, slapping, pain, submission, etc. are left in their basic modes which are in the main undesirable.
BDSM is one way of approaching the 'mind/body problem'. It can be very effective in its results provided those who employ it are of a psychological and physical fitness up to the task of managing experimentation in that it does not compromise beneficial perceptions both for one's self and for others.
Examples of mismanagement are an increasing inability to regard or express sexuality in ways other than BDSM, i.e., the power of the sexual charge exceeded the user's control of it. Extreme examples of this are where the user finds something of interest in torture and snuff films.
There are other details to this but that's a basic overview.