Boiled pork ribs don’t have much of their original flavor left, since most of it went into the soup. Duck on the other hand—especially duck fat—has such a dominant flavor that it can crowd out other, weaker flavors completely.
This little trick allows you to turn pork into duck by adding duck leftovers to a pork dish, in this case Roast Duck with Pineapple sauce and Purple Pig Soup #2. The broth superimposes the duck flavor on the exhausted pork dish to rejuvenate it. If you have pork soup, you can also boil the bones and carcass of the duck in the soup for a similar result.
This will work with any combination—some better than others— of a strong-flavored dish with an exhausted or nearly flavorless one.
The opposite is true as well.
If you want to preserve a delicate flavor—e.g. anise in fennel—avoid adding stronger flavors that could overpower the one you're trying to bring out.