I saw a TV show where a woman who was paralysed from the neck down - and so couldn't control her muscles to push - had a vaginal delivery. Her uterus did the work itself, although they gave her a bit of help with the ventouse right at the end.
Re the people yelling "puuuush", that always struck me as ridiculous and I kept laughing at my DH when he did it (thank you, epidural) as it seemed foolish that he would tell me what to do (when he never ahs before!). I'm sure there was a study in the last couple of years that said that "coaching" during birth made little or no difference to how quickly the baby was born.
I really loved that article on the other thread.