According to one view in Chazal the “karnei hod” which emanated from Moshe’s face came from the bit of ink left on the quill after he finished transcribing the Torah. Ksav Sofer beautifully explains that what Chazal are telling us is that not everything in Torah is or can be contained in the text. There are secrets that were never set down in writing – the leftover ink that never made it onto paper – that Moshe, who learned Toras Hashem directly, was privy too. Moshe covered his shining face with a mask, meaning he hid these secrets behind a veil, so that only those who would put in the time and effort to dig deeper would discover them.
The point the Ksav Sofer is making applies not just to mysterious “sisrei Torah,” but to Torah law in general. The thinking of a talmid chacham cannot always (can it ever?) be reduced to a series of logical steps or a precise formula that can be mapped out on paper – one does not take a corpus of texts, add deductive or inductive logic, and poof, generate an answer. Feel for a text’s nuance, for whether an interpretation is too narrow or too broad, for how rules apply to a given situation etc. is something that a seasoned scholar feels in his kishkes, for lack of a better term. What constitutes sound, informed judgment it is not something that can be transmitted through writing, but must come from the experience of a lifetime of immersion in Torah study. That is the drop of ink left on the quill after all that can be written has been.