In short, Bill Parry's theory is that there is some involuntary misfiring of the valsalva mechanism. This misfiring occurs when we have the urge to "try hard". In his eyes, this misfiring explains why he can talk to his dog, but cannot talk on the phone. Or he can be fluent talking at a restaurant until the server comes. Some of the muscles included in the valsalva mechanism are your stomach, larynx, throat, mouth etc. I read his book 15 years ago and I sat in a couple of workshops. I think it's an interesting theory. I remember one of the techniques that helped me was keeping your "valsalva muscles" loose. I mentioned "loose rectum" to my wife one day and it became an inside joke.