Mike Garofalo writes about Mind-Body Arts, Philosophy, Nature, and the Eight Ways.
Maybe the answer is in the first comment of this post: http://taichinotebook.wordpress.com/2011/05/27/tucking-the-tailbone/ " You might consider that tucking approach as a way to work against often seen male-alignment in the shown body shape of modern men..." and... "“Tuck the tailbone” has to do with what naturally happens when your opponent applies force onto a correctly aligned tai chi posture. It is something that you cannot do yourself. Your opponent does it for you. " http://www.mastergohring.com/waters.htm
I find that when doing bow stances my lumbar area is tucked in, groin pressed forward, iliopsoas streched (extended) on the back leg side and contracted (flexed) on the front leg side. There are lots of "bow" stances in Taijiquan. Root into the ground with the legs, and draw lots of power through the legs and waist. The tailbone seems "tucked" in these bow stances.