Coitus à mammilla is intercourse between a woman’s breasts (‘mammae’ being Latin for breasts). It’s also called “Russian Intercourse” of  “tit fucking”.
Normally the girl lies on her back, wile the guy leans over her, supporting himself with his arms. He thrusts between her breasts.
The girl pushes her breasts together with her hands, making sure the penis gets enough stimulation by controlling the pressure.
Use lots of lubricant, otherwise it the penis doesn’t slide very well.
When a guy comes this way, he gives the girl a “pearl necklace” (slang).