Thank you both for your replies. I checked the Adagio Fundamental BCH video but it was not mentioned there. Stefan, your comments gave me some insight to why I am asking this question to begin with.
Greg likes to use the balloon as a demonstration, as you know. He inflates the balloon and then, citing “passive reduction”, allows the air to escape using only the energy supplied by the elasticity of the balloon. He just lets the air out until the pressure equalizes. The ‘chest-up’ exhalation does not match the balloon. With the chest up on exhalation, it is as though the body is acting like a bellows…’squeezing’ with the abs while holding the chest up. That seems a lot different to me than letting the stretched-out intercostal muscles just relax, like letting air out of a balloon. Or perhaps I am overthinking it. Anyway, I at least understand my own question better and can explore how it works for me. thanks!
cc