A verse about that which was concealed is understood to mean either that secrets of Torah were hidden away - as in details of Creation, or Ma'aseh Merkavah, or perhaps the meaning of mitzvot altogether. Note that God rejoices when He is "conquered" by people. For example, when Moshe convinces God not to destroy Bnei Yisrael. Also, more on Hallel. But with a twist - that God is rejoicing in us. First, Avraham, Yitzchak, Yaakov, on down, each refusing to accept that they are deserving of His honor, until David - to whom more of the Tehillim are ascribed than anyone else. Plus, afikoman, which seems to be a delicacy that is eaten after the meal (of the Korban Pesach). A completely different meaning from our afikoman today.