As long as you are okay, you just get out. You will be amazed what you can do even when in advanced stages of labour. I was walking up and down my stairs in between pushes easily. If for any reason you were to lose consciousness, there are two midwives there (who are used to lifting people if needed) and your partner and possibly family or a doula. They'll get you out just fine. Slitting the pool would be a pretty dumb approach to an emergency. Besides soaking all the equipment they'd need to deal with said emergency, it would take longer to find a knife and cut through the pool (they are made of pretty tough material!) than to just lift you out. However, I'd assume that's a pretty rare situation anyway. The kinds of emergencies that happen quickly are rare in birth. A good midwife can see any complications coming a mile away and would get you out of the pool before anything happened that required doing anything like that.