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.
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