A lot of births happen while women lay on their back, even though a vertical posture is much more natural. A birthing stool looks like a toilet seat, but a little bit lower. Because you are sitting lower more room is created in the pelvic area, which can help during the birth.
Pushing is usually easier and you can control it a little bit better compared to a horizontal posture. The posture makes the risk for a little tear bigger, but chances on a total rupture less high. A little tear usually heals very well. All obstetricians in our practice are experienced in guiding a birth on a birthing stool and we always bring it, so you do not have to make a reservation to use it.