Lesson 16: Custom Morphs & Magnets (renderosity.com)

You could look at the above link to learn about morphs and magnets.  The basic functionality has not changed that much.  Trying to explain it will be hard if you are not familiar with how Poser works, and it is best t try and follow along.  In a nutshell you will either use the Morph brush to move the part of the chest out (the expand of breathing) or use a magnet to move the parts out, and then you will have a dial that you turn up and down along the timeline to make appear to be breathing

Poser 5, 6, 7, 8, Poser Pro 9 (2012), 10 (2014), 11, 12, 13