The reference I have for the above is  Minami J (1985) Helping mothers with
Letdown. Oregon Areas Leaders' letter of La Leche League.

Sandra Lang, Senior Lecturer in the Department of Midwifery Studies at the
University of Central Lancashire, UK,  who has written a lovely book called
Breastfeeding Special Care Babies devotes a good few pages to back massage.
She writes "... it is suggested that this may help stimulate the release of
oxytocin and the 'let-down' reflex."  She sites the above Minami J reference.
Yours, Helen M. Woodman, National Childbirth Trust Breastfeeding Counsellor,
UK.