I think the theory behind sterile water injections is that they hurt like a b*st*rd! My sis is a midwife and she said it's amazing how much help they are to women having posterior babies. Apparently they are comparable to a wasp sting.

Physiologically I think they work (regressing back to all those physiology clases now...) because the sudden and intense superficial pain of the injection is instinctively considered to be more of a threat to your safety than the internal pain being experienced. The external pain could be a predator that has just taken a swipe at you, therefore triggering your brain to regognise that pain as the more important one (even though it is a less severe pain) and blocking out the internal pain.

That's my cr*p nurses attempt at expaining it anyway, there are probably midwives on here who could do a much better job!