Yes, this is a very old thread, but I have to correct and explain several things here.
#1. You do not twist flexible pvc pipe when gluing it into fittings. It has nothing to do with bubbles. The reason why is that when you cut flexible pvc pipe you will note it never cuts perfectly straight. There is always a little bump on the end. If you were to twist the flexible pipe what happens is that the little bump acts as a swipe and REMOVES glue (and pvc plastic) creating a spot behind it with the glue (and pvc plastic) are being swiped away. And it does so at the most critical point in the glue joint: the base of the socket.
For the real reason why, keep reading.
PVC pipe and fittings are what is called an interference fit. The socket is tapered so that at the base (bottom) of the socket the diameter is actually smaller than the pipe that is being inserted into it. That is why when you dry fit pvc pipe to the fitting it only goes in about 1/2 way w/o a lot of force. When you glue the pipe into the fitting the solvent in the primer "melts" the pvc plastic in the pipe AND the fitting so that you make a good joint because "no two solid objects can occupy the same space." The solvent is making the pipe and fitting NON SOLID at the moment you make the joint. So by twisting the pipe that little bump on the end of the flexible pvc pipe swipes and REMOVES pvc plastic to another point in the joint creating a "divot" in the wall of the socket behind the bump. That divot will be a weak spot in the glue joint and could cause a leak.
You are never to twist flexible pvc pipe when making a glue joint. The reason rigid pvc pipe is twisted is to assure an even spread of the glue. Which is why I made our video
https://www.youtube.com/embed/QITVd4N7064 to show the correct way to make a pvc glue joint. With rigid pvc pipe you can be a little lazy with your glue application and twist will help recover your mistake. HOWEVER if you cut rigid pvc pipe crooked or at an angle you do not twist because it'll do the same thing (remove material). So if your pipe cuts are not perfect or you are using broken pipe, do not twist it.
Yes, lots of people can do these things incorrectly and not have a leak. That is because the system is engineered to take many of these things into consideration. The bottom 1/3" of the socket is where the joint is really sealed and as long as you have a continuous joint all the way around it, you should be okay.
#2. Barb or insert fittings. You never use barb or insert fittings with flexible pvc pipe. The spiral will actually start to unwind itself and over time due to the difference in ID of the pipe between that which is over the fitting and that which is not and the joint could fail. (Yes, if you have a clamp on it that will keep it from further unwiding.) However the pressure in the pipe is acting to expand the pipe and pull it away from the barb, thus compromising the seal. Also it's very hard to get the insert fittings into the pipe. If you want to use barb/insert fittings use vinyl tubing, the fiberglass reinforced or hybrid hose. Those products are engineered for use with barb fittings.
#3. "Flexible pvc pipe glue" .... there is no difference. It's a marketing ploy to get you to buy another can of glue. We recommend the clear or blue glue, medium or heavy bodied. We do not recommend the gray because it drys too hard (and becomes brittle over time.)
My qualifications:
Gluing flexible PVC pipe since 1972. I've made well over a million glue joints. I've been in the swimming pool, hot tub, plumbing, aquarium and irrigation almost the entire 45+ years of my career. I've fixed thousands of leaks in pvc joints. To see one of my greatest secrets on how to do so, see this video
https://www.youtube.com/embed/wo-R0oqnMpc I made a couple years ago.
Chief Technical Support for FlexPVC.com since 2002.