訣

A phono-semantic compound composed of:

言 — semantic component meaning “speech; words”;

夬 — phonetic component indicating the sound (결 / jué). The original sense relates to spoken words at the moment of parting, which later extended to important spoken instructions or secrets.
Semantic development: - words spoken at parting; - to say farewell; to separate; - secret method or essential formula.

결별 (訣別) — farewell; parting

비결 (祕訣) — secret; key method; know-how

구결 (口訣) — mnemonic formula; oral instruction

The meaning “secret method” comes from the idea of important knowledge transmitted orally. Today, 訣 is most commonly encountered in compounds like 祕訣 (비결) meaning “key to success”.
Related characters: 別 — to separate; to part

秘 — secret

言 — speech; words