Hexagram #21 is called “Gnawing” or “Biting Through”. It points to a difficult situation, and likens it to something being in your mouth, keeping your jaws from coming together. You are advised to attack the problem, to gnaw on it until you break through it. Once you break through it, you will also see what is hidden in this difficulty. It reminds me of the old Tootsie Pop commercial, where the wise Mr. Owl bit into the Tootsie Pop to reveal the Toostie Roll center.
The trigrams that make up this hexagram are fire above thunder, which suggests lightning and thunder — a vibrant action with far-reacting effects. The nuclear hexagram is #39, Difficulties. This repeats the idea that hidden you can find something hidden within the difficulty you are facing.
The first changing line says that you cannot move for now, and cautions you to yield to the situation for now. The second changing line supports your enthusiasm in attacking the difficulty and says you should keep going. The third changing line says that as you bite through, you will encounter something that needs to be rooted out. Do not merely adapt to this situation. The fourth changing line advises you that there is something of worth in past efforts, and for you to find that and use it in the current situation. The fifth changing line says that in this difficult task you will have to encounter something about yourself. but the rewards will be worth it. The sixth changing line admonishes you that you aren’t listening, and you have blocked yourself off by this action.