The sexual act in which one partner lays on the ground face first and proceeds to stretch their butt-cheeks apart revealing the anal cavity. Their counterpart must be a lactating woman; she squeezes her breasts into the anus, causing the nutritious milk to pour inside. The milk mixes with remaining fecal matter creating a delicious snack. She then kneels down and felches the milky shake out of the anus.
After hours of exhausting fornication, Jennifer decided she could go for a sphincter shake fresh from her lover's anus. Hopefully she doesn't wake the baby.