Because Really You Are

On Page Sixty,  the question is odd, it's "Some say that while practicing mindfulness of breathing the soul goes out of the body."

Sayadaw gives a two part answer;

First, because of the nature of the mind, almost anything can happen.
Second, if you realize the nature on mentality/materiality, you can solve the soul problem, the idea of a soul arises because of the perception of compactness.