Wu Jianquan of Wu style Tai Chi demonstrating how to centre the coccyx perfectly.
People often get very confused about “tucking the tailbone” in Tai Chi Chuan. It’s a posture requirement that’s mentioned in the Song of the 13 Postures, where it says:
“When the tailbone is centered and straight,
the shen [spirit of vitality] goes through to the headtop.”
The tailbone is what’s more normally known as the coccyx. Notice, there’s no mention of “tucking” here at all, just keeping it centered and straight, which makes me wonder where everybody gets the idea of tucking it from? It’s quite a crucial misunderstanding because the word “tucking” implies bending the hips and tucking under, which results in an unnatural and distorted posture that isn’t comfortable at all. You can’t even stand comfortably like that, let alone fight!
I find that the best way to achieve a centered coccyx is to first bend the knees (you can’t ‘centre’ you coccyx with straight legs) and then relax the lumbar region of your spine. You’re looking for a natural feeling of lengthening. A dropping down. Once you’ve got that feeling then your coccyx will be in the right place. You shouldn’t feel any of the muscles or tendons in the backs of your legs being activated when you do this either, so look out for introducing tension there – it’s usually a sign that you’re starting to “tuck”, rather than lengthen the spine.
The Tai Chi Notebook 
Your right and its like a chinese whisper that has become gospel to most schools which cause the opposite of what Tai chi is supposed to achieve ;Relaxation. Ask anyone who does this and they can’t reallyexplain in an anatomical way they refer to chi and I watched a teacher putting her students in that position with the obvious result that they lean back she then pushed them forward. The tension on their faces said it all. Madness and the sad thing is that the good students who practise the most become unhealthy.
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Hi Bai Yiming,
It’s a good point that if an individual student has a particular problem of “sticking out the bottom” then some advice to tuck it might be appropriate. Unfortunately too often I see a room full of Tai Chi students who have turned themselves into upright versions of a human tortoise in an effort to ‘tuck’ their behinds! 🙂
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You might consider that tucking approach as a way to work against often seen male-alignment in the shown body shape of modern men, with the buttoms sticking out. If you don’t have that problem, you can keep the coccyx naturally vertical, alright. But if you adopted an unhealthy position over time, Chinese teachers let you work intentionally against it by tucking. After some time of reajusting, you might find back to a natural vertical position of the end of your spine.
My 2 cts
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