The next four lines of the Xin Xin Ming continue the thread of non-duality. I translate it as follows:
If there is one hundredth of a hair difference
Heaven and earth hang apart, separated by a barrier
Wanting reality to appear presently
Do not settle for following or going against
There are a few things to highlight in this segment. Discriminating, or creating difference itself imposes a barrier between heaven and earth, mind and body, subject and object. It is not that discrimination causes duality, rather, discrimination is duality.
Compare this to Zhuang Zi:
The knowledge of the ancients was perfect. How perfect? At first, they did not know that there were things. This is the most perfect knowledge; nothing can be added. Next, they knew that there were things, but did not yet make distinctions between them. Next they made distinctions among them, but they did not yet pass judgments upon them. When judgments were passed, Tao was destroyed.
As noted before, in Taoist writings the path of decent and ascent are the same path with different directions. Accordingly, if one wanted to obtain perfect knowledge, one would stop passing judgments, then stop making distinctions, than stop making things into object.
The Chinese character for wanting, 欲, is a bent, open mouthed figure. The idea is that this figure is empty, wanting to be filled. This is the perfect image of desire. There is an open space, our own inherent emptiness. Rather than accepting this, we seek to fill it, to solidify it, to become something. The rest of this couplet implies that reality will appear before our very face out of concealment. Again, it is not that we are gaining something, but that we are losing our discriminations, and from this reality appears naturally.
The next lines can be puzzling. It says not to follow or go against, to neither go with nor oppose. There is a lot of talk in ancient writings about following the Tao, or going with the flow or current of the Tao. The Xin Xin Ming takes an unusual stance here. We should not follow the Tao, or go against it. This itself is a discrimination, and therefore a barrier. As we learned from Lao Zi, dualities create one another, and apart from one another have no meaning. To follow the Tao, to oppose the Tao is another set of dualities.
This does not need to remain an abstract, philosophical idea. When we see something, initially we just see. When we hear, there is just sound. There is no view in the seeing or the hearing. The view comes after. I see the ocean, I am here it is there. I hear a bird. The bird is there and I am here. But if we look to raw experience, there is none of this. Reality just is. We impose a screen over it with our discriminating minds. Removing this screen, it appears as it appears.
Some people claim that non-duality is a form of nihilism, or believing in nothing. This is not what the Xin Xin Ming is proposing. Rather, it is proposing a radical acceptance of things as they are. Why radical? Because it goes against commonsense perceptions. People tend to think in terms of this and that, subject and object. Accepting both as they are, there is no subject and object, but rather a continuous subject-object.