The diaphragm call is a great tool allowing you to call turkeys effectively yet maintaining full use of your hands for hunting.  The diaphragm fits halfway between your front teeth and back of your mouth.  It can be bent if it feels too big and doesn't feel to fit your mouth.  It should fit comfortably against your palate.  The beter the call seals against your mouth, the better your clucks and yelps will sound.