Sensual sucking involves a complete absorption of the attention and leads either to sleep or even to a motor reaction in the nature of an orgasm. It is not infrequently combined with rubbing some sensitive part of the body such as the breast or the external genitalia.
Freud establishes sensual sucking as the paradigmatic infantile sexual manifestation, auto-erotic in structure, erotogenic in aim, and continuous with masturbation.
, Three Essays on the Theory of Sexuality, 1905thesis