For the same reason Chunyi would say to do your Qigong into the wind, rather having it at your back, Darcie recommends not letting C7, the tummy, lower back, and feet chill during and right after deep mediation/active exercises.
It's also similar to Chunyi talking about no deep meditation in water at this level.
You hear Chunyi talk about C7 in the Level II course as being the "key" to opening the upper part of the body. check out the section on the head.
Much Love, Shawn