Rather than be "frustrated," you might say to yourself you haven't taken the teacher's curriculum and may be looking at only a small clip. "Sink the chi" may be an abbreviated description of a process that is explained further within the course
I have no idea who you are talking to. I know what it means. I am frustrated to the misuse of the term.
About the Sun Lutang quote:
"When I roused the energy in my abdomen, I could throw an opponent some eight or ten feet away. Whether walking, standing, sitting, or lying down, at any time it was thus. I thought that by accumulating energy through sinking it down, I would likely attain the art’s internal power, and that those who were unable to sink energy to their lower abdomens were all of the external school.
...
Song said: “Breathing is divided into internal and external, but in boxing arts there’s no distinction between internal and external. If you are good at nurturing energy, then it’s internal. If you’re not good at nurturing energy, then it’s external.""
Didn't remember the details in this quote though I have used it myself in a couple of posts. I checked the original text and sure Sun use the word "qi". Brennan has translated qi sometimes into "breath" and sometimes to "energy". However, I agree with most of his translation here (though some of the subtleties are lost. But that's the only way to translate this kind of text. You must always choose a level of meaning and stick to it. ). The thing is that in Chinese, some words are exchangeable, just as "qi", which is often used to mean breath instead of "energy". When Yang Zhenduo or Sun Lutang say "sink qi" I believe that what they mean is to sink the breath and do not refer to the "energy" "qi". But again, sometimes teacher can mean both qi and breath at the same time, because and again, the concepts are exchangeable. And so again, it's better to understand these things from a practical pov, gaining personal practical experience and not be too concerned about the theory.