Human body was created and evolved by nature and nature is the body of God. When we make a move in Tai chi, there are some basic rules to follow.
Take a look at Master Yang chengfu's posture on the left, you may notice that though he leans forward relatively but his back is still straight.
Also, his coccygeal verterbra never goes over the vertical line up from the left heel according to this picture.
Oops, I almost forgot the most important part: The cervical vertebra ( head and neck ). As you can see, Master Yang's cervical vertebra doesn't go bend as the body leans forward. It's still on the same line with the back.
You may not see his left arm's position, but I can tell you that his left elbow is vertically pointing to his left knee.
Now check the right hand. The index finger is horizontally pointing to the nose.