口開神氣散 When the Mouth Opens
他不敢答應者何意？想著：「口開神氣散，舌動是非生。」卻又一條心兒想著：「若死住法兒不開口，只怕他心狠，頃刻間就害了性命。」他不敢答应者何意？想着：「口开神气散，舌动是非生。」却又一条心儿想着：「若死住法儿不开口，只怕他心狠，顷刻间就害了性命。」
逐句解 Line by line
And why did he dare not answer?
The narrator turns to the audience and asks the question for them. It is a storyteller's move, not a novelist's, and this book keeps it because it was written to be performed. 應 is yìng, answering, once more.
He was thinking: "Let the mouth open and the spirit-breath scatters; let the tongue move and right and wrong are born."
A matched pair of five-character lines, and both halves of it are proverbial. 神氣 is the vital breath-spirit that Daoist practice conserves, and 散 is to scatter or disperse — speech leaks it. 是非 is literally is-and-is-not, and it is the ordinary word for trouble, disputes, the mess that talking gets you into. Here 著 is zhe, the reading that holds an action open, so he is not thinking once but sitting there thinking.
But then another strand of his mind was thinking: "If I hold dead still by the rule and never open my mouth, I am afraid her heart is cruel enough to take my life in a moment."
The image in 一條心兒 is a strand or thread of mind, as though the heart-mind were made of separate lines of thought — which is exactly the state being described. 死住法兒 is a vernacular for holding rigidly to a rule, sitting tight. Again 著 is zhe, the thought sustained rather than struck.
This is the chapter's most interior passage and it is built out of two proverbs pulling against each other. Silence preserves you; silence gets you killed. Neither line is original and the collision between them is.
The scattering breath-spirit is not a metaphor in the tradition the novel is drawing on. Daoist inner practice treats the vital breath as a finite quantity that leaks out through the senses and the mouth, and holding the tongue is a real technique, not merely good manners. Tripitaka is applying his training and finding that it may kill him.
The novel gives him two minds and lets both speak. It is a strikingly modern way to stage indecision, and it is done in about forty characters.