separation, part, secret
訣 centers on a decisive parting or separation, often with finality. The 'secret' sense extends from the idea of a private, parting word or a key piece of knowledge shared at a critical moment of departure.
訣 combines 言 (words) with 夬, which likely contributes the sound and the idea of parting or dividing. The character suggests parting words or a decisive verbal separation.
Words (言) plus a shape associated with parting (夬) create the idea of a final farewell or a secret shared at parting.
For ケツ, imagine a secret parting where you hand over a key (ケ) and say 'tsu' (ツ) as a final code word: key-tsu -> ケツ.