They are like a tightly wound spring that goes inside the pipe and holds it from collapsing when bent. IMO it is a not very useful unless you have knees of steel. It is a much better bend to use a hot bender.
About a year or so ago I purchased a 1/2" and a 3/4" PipeViper made by Kraloy. They have worked well for me. You can't use them for every situation but there have been times when I was glad I had one.