An angle of 45° between neck and back in partial hanging achieves the greatest fatality rate of over 98%. Thick knots in the ligature positioned over the carotid arteries can cause unconsciousness after ten seconds. Once unconscious, without outside intervention death is inevitable within a few minutes.
I think that most people who kill themselves in that way don't really mean to, and don't realize how deadly a method it is. Maybe they have just enough time to realize that they've just killed themselves, in the moment everything very quickly goes dark and they black out and their whole weight goes onto the slip knot.