This French book about marital continence, Art D’Aimer, triggered a tempest among French Catholics when it appeared in 1949. The Synergy-like practice was also known as the étreinte réservée (reserved embrace)
Paul Chanson proposed conjugal continence as a birth control solution for couples with legitimate reasons for limiting births. He also touted it as a way to promote sexual harmony between spouses. The Catholic Church soon caused the book to be removed from circulation and suppressed its publication and translation.
The links below go to the scanned pages of the original text of Art D’Aimer:
Epilogue – “Art D’Aimer et Vie Spirituelle Chrétienne” by H.M. Féret
Chanson, Paul, and H.M. Féret. 1949. Art d’aimer et continence conjugale. París: Familiales de France.
See also The Chanson Affair (1950-1952)
L’Étreinte Réservée: Témoignage des Époux by Paul Chanson (1951)
What if my family had known about Chanson’s “Reserved Embrace”?
Those who read French may also enjoy Karezza, l’art de l’amour : la voie de l’extase sexuelle : un tantrisme occidental by J. William Lloyd (1931).