I'm working on a class themed around fall. Lungs are yin. Large intestine is yang. I found some new and different poses for targeting the large intestine meridian, but stretching in a yin way (i.e., held for 2-3 minutes). I was taught that stressing yang tissues in a yin way (and vice versa) is unsafe. Is it?
In general, we don't want to apply yin stresses to yang tissues nor yang stresses to yin tissues: but this is a short hand way of saying "Do not place long held stresses (yin) on the muscles (yang) and do not place dynamic, rhythmic stresses (yang) on joints and ligaments (yin)."
The organs are not yin and yang in the same way that ligaments and muscles are yin and yang: the contexts are different. It is okay to stress yang meridians in a yin way: that is what an acupuncturist does: the needles are left in the acupuncture points along yin and yang meridians for a very long time.