Irezumi: Spirit of Tattoo (1982)

To satisfy her lover's fetish for tattooed women, Akane agrees to have her body covered in eloborate tattoos by Kyogoro, an old craftsman. Kyogoro has developed a special technique, by which his assistant, Harutsune, keeps Katsuko's mind off the pain while Kyogoro does his work. After the work is done, Harutsune makes the shocking discovery of how Kyogoro mastered his technique.