We have an upper body and a lower body, split at the sacrum. This becomes clear with practice. But we also have an upper mind and a lower mind. The upper mind (head mind) has become the thinking mind and associates with the upper body. The lower mind (belly mind) is the primordial gut-feeling mind associated with the lower body. Sinking in taiji (getting through the legs and into the ground) is difficult because it requires the grim subterranean seriousness of lower mind.