Regarding questions about the alignment of the spine in Tai Chi, below is a statement taken from a longer article on my website.
Read the full article at http://www.keithroost.com/keiths-blog.html
In the Golden Rooster School, Wu Dang Tai Chi Chuan practice involves the correct alignment of the spine to facilitate the optimum bio- mechanical use of the body commensurate with the movement or action undertaken.
Therefore in Single Whip we move the hips to a mid position to lengthen the lower spine, facilitate opening the hips, and align the spine to support the rotation of the waist and issue and consequent absorption of forces through the use of Fa Jin.
Different movements may require different alignment or indeed several different alignments throughout the technique
Keith has studied the Chinese Internal arts for over 40 years. He lives in England and Portugal with artist, designer and writer Gloria Dean and teaches in Portugal and the UK.