Sissy training is a methodology in which submissive men learn to take on traditionally feminine roles. The submissive, known as a wimp and a loser, learns under the guidance of his dominatrix to adopt extreme female behaviour and perform female activities such as being a maid.
It is particularly appealing for a dominatrix to force her sissy, who is usually not gay, to have sex with men in front of her or to present the sissy to other women for her shame and humiliation.
As a sign of total submission to their goddess, the sissy's penis is usually locked in a chastity, which further increases their lust and dependence.
Examples on video how to transform a man into a sissy can be found here: Sissy training!