Different Breathing Forms Emerge During Different States

Whoever meditates enough times, notes that different forms of breathing emerge at different times during the course of the meditation.

Less desire to exhale emerges as one exhales methodically, as I discussed before in the last post, inhalations mirroring exhalations kind of thing. This situation coincides with decreased pressure, that is one becomes more and more empty from pressures.

 

As one says mu inwardly, while exhaling in the four phases discussed in the previous post, to be methodical a little bit, as the author recommends, one may also add pressure to the mu, so that more pressure is placed on the lower abdomen. This appears to be what the author is suggesting, but I don't really understand it. Does one push the lower abdomen forward as one exhales while saying mu? It is not clear to me.