Ive heard that it's when the doctor separates the water sac from the cervix. Some say it helps induce labor and others say it doesn't. One post I read the lady reported going into labor within 12 hours, others say nothing happened. And I think timing depends on your doctor. I wouldn't think they would do it before 39 weeks though
I had mine done twice. The first doctor barely scrapped it and nothing happened. The second doctor scrapped it so hard, and because my daughters head was right in my pelvis, I think she felt it and reacted. I ended up having to be monitored for 20 minutes because it shocked her and her heart rhythem changed. Having said that, I lost my mucus plug about 15 hours later and went into labour. The second scrapping was very painful but she was 6 days overdue by then so I wanted to get things started.