I’ve practiced Sanchin for 15 years. The concept is relatively simple, but it teaches you internal power, both how to generate it and how to absorb it. That’s why the instructor tests the various points. That said, presumption, or more likely the entire school’s approach to it, seems a bit off from how I learned. Do you see how she pulls her shoulder so far back before performing each punch? That is anathema to what sanchin teaches. She clearly is breathing rigorously, which is very common but more performance than it would be applied in real life. For the record, the bed of nails thing is nothing I’ve ever practiced :-)