I agree with a lot of the replies where guys use the mouth calls when the bird is in close to reduce movement. I can also get a lot better sounds out of my mouth calls when trying to be aggressive than other calls. As far as what is a friction call...they are also refered to sometimes as slate calls because a lot of them have slate surfaces, they are a surface of glass, slate, copper, aluminum, or many other materials fitted into a pot and you would use a striker to make the sound on them...love them!!