(Here are the pictures from the Back Pain page again - so you can compare the shapes)
Why the forward sloping seat?
Unlike most seats posChair 1 slopes forwards. This does two things to help the pelvis stay upright:
It lets the thighs slope downwards. For most people bringing the thighs up to horizontal, as most chairs do, tips the pelvis back. (This may be caused by shortened hamstrings, and sometimes by tight clothing too.)
It also exerts less pressure under the thighs, so they don't support much torso weight (another cause of slumping). This makes sure the torso weight is supported directly below the spine.
Why is it on a rocker?
The rocker is to encourage balance and poise.
We have a wonderful sense of balance for standing and walking, which we take for granted because we find it so easy. But we seldom get the chance to balance when sitting.
It is easier to sit upright when you are balanced. It is relaxing yet invigorating at the same time. It is easier to be alert and attentive than when your body is allowed to become lifeless against a backrest. And when you are alert and attentive it is easier to sit upright.
Even when sitting still you feel the freedom to move. (You will always be making imperceptible movements to maintain your balance, and these will keep your balancing reflexes working at their best).
As long as the pelvis remains in balance it can support a lot of bodyweight. It takes very little muscular effort to keep it upright because the support is directly below where the weight presses down. It is not true that sitting upright needs strong back muscles. If you are using much effort you are sitting inefficiently and will tire your muscles.
You do not need to strengthen your back, only to take away the forces that tip you off balance.
Many people find they sit like in Fig. 2 without trying, and find it extremely comfortable. This is because it's a healthy natural spine alignment.
For a healthy back this is the single most important thing you can do to keep it healthy.
For an injured back this stops the cause of most of the damage, and allows healing to begin.