Spine and Knee Alignment
During most of the movements, drop the tailbone and extend the spine, as if a string was gently pulling on the crown of your head and tailbone. This helps gently stretch & align the back, helping to remove back and shoulder tension. Benefits for the back are one of the most reported benefits of Tai Chi, and straightened my own back from scoliosis. Definitely something to talk with your doctor about!
As for the knees, some forms of martial arts, particularly those focused on competition, feature wide low stances. This type of stance looks very powerful, but can place a large amount of stress on the knees, as well as result in a higher more aerobic level heart rate. This can cause damage to the knees over time, and also affects potential metarobic benefits. Traditional styles of Tai Chi may also turn the lead foot inward, to help protect the groin during combat. But over time, years and decades of practice, this inward turn may cause knee strain, due to unbalanced muscle development.
For most people, to enhance benefits for health, particularly metarobic benefits, the active leg should be bent slightly, and never to the point where the knee extends beyond the toes. The foot should be straight, in line with the knee. This can help foster even development of the vastus medialis muscle, which can in many ways act as a natural knee brace. Strengthening the vastus medialis has been associated with improved knee strength and pain in osteoarthritis patients. As noted in the introduction (of the primer), in my youth I had to give up running, due to swollen and inflamed knees. These exercises strengthened the muscles around my knees, and saved me from knee surgery. However, individual cases may differ, and if you have knee problems or concerns, then this is something to talk to your doctor about. Variety is the spice of life, and mindful exercises provides one more tool to use for optimal health. I have found that biking combined with my mindful practices helps my knees tremendously!