This is not sick, but a way for a tense mind to release. The safe use of this technique is fine. The cock ring is one form of bondage, Chinese girls feet, etc. Violence and injury are to be avoided and are not part of a smart master slave relationship. Every relationship is a balance between these two extremes whether it is a love affair a marriage or another relationship of power like work, the courts, schools, etc.
Teachers are famous for being sadistic.
And so are many parents unfortunately. These all fuel the desires and fears and wants and needs that each of us have and this in tern is expressed in sexual relations weather real or imagined. Even the idea of a punishment is exciting. The feeling of being free of guilt for that one second of the crack of the whip. The slap or the hit.