How does one do mindfulness of breathing with relaxation?

In this lecture , by B. Alan Wallace, of mindfulness of breathing with relaxation, it has been instructed that I have to release all thoughts, relax every part of the body and let go of all involuntary thoughts during exhalation. Now what exactly is meant by releasing all thoughts with exhalation? What should I do during exhalation? Does it mean that mind will go blank during exhalation? If yes, then how is it possible?