This thread is over a year old, but in case anyone comes here, be advised so you don't have to learn the expensive way. There are two types of correction bit. The inverted "U" shaped mouthpiece is popular, but it DOES NOT move in all directions as suggested by one poster. For this you would need a hinged bit, I.e. A square-shaped port. This allows you to lift the rein on one side and only one side of the shank goes up. Try this when shopping. The shanks go up in unison because the "U" doesn't split in the middle. Myler (or Toklat, if you want to save some money by buying the approved Myler 'rip off' maker) makes ever lower ported bits that have a metal tube in the middle that holds the two sides together, thereby allowing rider to life only one shoulder or the other. Check this out for yourself before spending $70+ every time, trying to find the right tool. Good luck!