SMG

Scintimammography or Screening Mammography (SMG) is a breast examination technique where a woman's breast is squeezed between two compression plates.
In pornography and BDSM, SMG is bondage of a woman's breasts, often with ropes (Shibari) but possibly other homemade or store-bought items, where the breasts are cinched rather tight, causing them to become more sensitive and darker, purplish in color.

When my girlfriend and I are playing with ropes and I am tying her up in a bikini chest harness, she wants me to wrap the ropes around her boobs pretty tight. She loves the feeling of SMG as much as I enjoy doing it!