[Lotus]

There are many types of receptors in breast tissue, estrogen receptors (in particular ER-A/Estrogen Receptors Alpha) is a nuclear receptor thats involved in the elongation of breast tissue, while ER-B (Estrogen Receptors Beta) helps to fight cancer amongst other functions. Progesterone develops ductal side branching, I can vouch for that one. 

Estrogen Receptors Alpha (ERα) and Beta (ERβ): Subtype-Selective Ligands and Clinical Potential
https://pmc.ncbi.nlm.nih.gov/articles/PMC4192010/

The method I describe (pinch and hold) is for stimulating the receptors in the breast tissue. Breast massages are important for breast health and growth (e.g. in the prevention of fibrous breast tissue). In my experience using this method it helps the breast lymphatic drainage system, like taking out the trash stored In breast tissue… (e.g. freeing up a clogged milk ducts).
This study found using compression therapy was an additional benefit for women with mild lymphedema;
https://link.springer.com/article/10.1023/B:BREA.0000032978.67677.9f

“Massaging your breasts helps the lymphatic system and can manually relieve the lymph nodes that drain the breast,” 
https://health.clevelandclinic.org/breast-massage