Spine in relation to the rules of Tai Chi

So I was wondering if anyone could let me know about the spine not moving, I am under the impression that if your feet do not move(not walking or stepping) then the spine from the bottom of the tail bone up to the top of the head stays in a straight line it. It may elevate through the bending and straightening of the legs, it may rotate through the turning of the waist, but it never ever moves. Is this the correct thought in all cases? Are there exceptions? of course it changes location when one steps but it does not bend? let me know what is what and thanks!!