This is one of the crucial point of a course that I did in Italy and improved my fluency and helped me to manage any situations. They call it literally "the presence". When you reach this experience, that we can also call the flow experience (in psychological language), you feel a deep connection with your audience, you want to bring the message out, not to keep for yourself. It's not easy to learn, maybe the easiest way to practice is to do some theatre course. Now, I can feel this sensation for example in stressful task, e.g. a conference, when I'm fully connect to my task and I cannot fail, and of course I'm prepared for that. I think it's not easy to find this presence in daily life situation when I'm tired or bored and I don't put to much effort in my communication