This Chinese term is translated in many ways. Usually you'll hear "sinking" or "relax and release". The Chinese character is said to look like pulling the pin out and "letting your hair down." I've also heard it as "open".
So what does that mean?
The easiest way to understand it is to look for the feeling of sitting in a chair vs. the feeling of doing a squat. With a squat you use your muscles to hold yourself up so you don't fall down. When sitting in a chair you let yourself go and plop down into the chair. That is the "sinking" feeling you are looking for. You don't want to plop down if you're standing, but you want to be light, relaxed, and not using unnecessary muscles and tension to hold yourself up.
If you are able, try both and feel the contrast.
In this video he talks about what happens with the arms as a result. As you sink, the arms begin to float up.
Enjoy.🙏