From my understanding, it's sort of like homomorphic encryption where you can create a function key based on a particular private key, which allows a certain function to operate on the encrypted data as if it was not encrypted. Thus, allowing for a note signing without revealing the nsec.
https://en.wikipedia.org/wiki/Functional_encryption#:~:text=Functional%20encryption%20(FE)%20is%20a,what%20the%20ciphertext%20is%20encrypting.