Can someone please go in to a bit of education on what has been called the grouper call and approaching a "Mouth Fill Depth" on descent and what is a proper sign of this and procedure for it. I am taking the PF course in October and it will be explained to us, but, would like to have some understanding of it and the why and how's before the class, I am not trying to learn off the internet this procedure let me state that, just curious before I am taught properly, thanks all.