I have had this done or done this with all three of my labors. In each case, I was about 9 cm with a big anterior lip and was stalling out in a great deal of pain. Something about the waters bulging seems to keep the baby's head from fully pressing against my cervix. I don't know if it's some sort of anatomical thing with my body or what.
With my first UC (second baby) this state lasted for about 4 hours of really intense pain. It pretty much completely resolved and the baby was born within 20 minutes of breaking the water. In hindsight, I totally wish I had broken the water earlier. The relief from the pressure was wonderful. In all three cases, the baby's head was well engaged in the pelvis and low and I was standing, thus lowering the risk of prolapsed cord. UC #2, I got to where I was starting to feel that same sensation I had felt in UC #1 and I decided to do it again (I had at this point a large anterior lip again). Baby was born 30 minutes after rupturing my membranes.
In both my UCs, DH has broken my water at my request. With the first one, he used his fingers to guide a pair of hemostats in and just caught the edge of the bag and pulled. We weren't prepared beforehand to do it, so that's why we did it that way. With my second UC, I thought it might be a possibility, so I purchased an amnicot from InHisHands.com for my birth kit.
The amnicot basically looks like a small condom with a tiny little nub on the end. You roll it over your finger and reach in and it snags the bag and ruptures it. DH just put it on and reached in with one finger and pulled once he snagged the membranes. It was easy because I had a bulging forewater. It worked like a charm and the nub that snags the bag is very small and dull. I was actually surprised it worked as well as it did. You could also purchase an amnihook for this purpose--looks like a crochet hook. That is what they usually use in the hospital.