Demonstrated by Master Luke Chan and the group in the June China training in ZhongShan.
From the top, the Baihui, of your head to the bottom, HuiYin, or perineum is your body centerline. This body centerline is an imaginary line because there is no such a line anatomically. But this body centerline, called Zhong Shin, is as really as it can be for those who have discovered it. For many of them, the body centerline exists as a sensation of strength.
In the ChiLel way, keeping your body straight means finding your centerline. Most people, especially the beginners, mistakenly regard their spine as their body centerline. This error becomes visible when they do the Chun Chi exercise.
To avoid this mistaken identity, you should imagine your body centerline as a tube going from top of your head to your perineum. When you squeeze in your arms during Chun Chi, you should imagine you are massaging your center tube. When you push out your hands, you should imagine the power is coming from inside the tube. If you have a chance to read the story #38 of the 101 Miracles of Natural Healing book, you will find Mr. Shu had discovered his body centerline and he got great healing results.