The talisman foot usually appears in the form of a pattern.
The Seven Profound Talisman Scripture mentioned that for a Low-Tier Talisman, the talisman foot is completely meaningless. It's essentially optional; leaving it out won't affect the talisman's efficacy.
The reason the talisman foot is said to affect one's ability to learn new talismans is because on High-Tier Talismans, the foot often serves as a form of ciphertext. In other words, a Talisman Maker uses the talisman foot to encrypt the talisman.
Once encrypted, one must spend a great deal of time decrypting it before they can possibly learn how to make it.
The lower portion of the Seven Profound Arrow Avoidance Talisman's core could, in fact, be considered a form of encryption.
