I've written a number of times about the pisiform bone. It's easy to locate. It's the size and shape of a hard pea, on the outer back side of your wrist as you do Xingyi style upright power standing. NOTE: it isn't the larger protuberant bone on the other side that is also kind of pea like, though bigger. I'm talking about the real pisiform bone which will be facing downward and outward as you stand in the pose above.
Activation of this makes a HUGE difference in your power harvest for this practice. Which is why I've written about it so much in the books. Here's a simple way to feel it: just mentally (99% mental! Don't tense up!) 'feel' that you are slightly pressing these two bones (one per side) toward the floor. No, this isn't just "sink your shoulders" or that normal stuff. Don't fuss with your arms at all, stand as shown above. Just 'feel' you are turning the pisiform bones slightly down, slight outward. This is a mental practice of engagement. I know this is too simple, plain and dumb for many of you. But for those with ears, give it a spin. Work to rule! And then you'll feel the Big Amp.