Castration involves applying a tourniquet of sorts, often an extremely constricting rubber band, around the neck of the scrotum in an effort to terminate all blood flow to the testicles and scrotum below the blockage. The most common approach is with a farm implement called an Elastrator, an inverted pliers-like tool used to apply a very small and extremely durable rubber band. This will cause tissue death and necrosis, possibly gangrene, and of course castration.
It is highly painful.
Castration, or the offer of it, is something I would consider to be an “ultimate” offering of submission and subjugation by and to the “master,” or dominant.