The "heart" here refers to the mind and thoughts of the person. Alternate translation: "so that you are not occupied" (See: [[rc://en/ta/man/jit/figs-metonymy]])
Just as a trap closes on an animal when the animal does not expect it, that day will happen when people do not expect it. Alternate translation: "that day does not happen when you are not expecting it, as when a trap closes suddenly on an animal" (See: [[rc://en/ta/man/jit/figs-simile]])