Sex therapist Claudia Six, Ph.D., recommends talking about what exactly each of you wants to try and what you want to get out of it in advance. To make sure anybody can back out at any time, it’s helpful to establish a code word completely unrelated to sex, like “watermelon” or “grapefruit.” (…)

If you want to practice physical restraint, Six recommends starting off with a sewing thread, since it’s easy to break out of but still gives the same feeling of surrender.

Glamour.com article