This is a reference to unusual sexual behaviors that were espoused by the school of Medina based on the idea that a wife's entire body was available for her husband's pleasure. Hossein Modarressi cites this reference to show that, along with other schools and jurists, Jaʿfar al-Ṣādiq strictly forbade such sexual relations.
This source is part of the Online Companion to Hossein Modarressi, Text and Interpretation: Imam Jaʿfar al-Ṣādiq and His Legacy in Islamic Law (Harvard University Press, 2022).