心機 is the working of the mind, its spring, and 一転 is one complete turn; the dictionaries keep two things, an occasion and a whole change of feeling. The word has no ancient source. It is a Meiji coinage, in print by 1888 and taken up by Kōda Rohan in a novel of 1891, at the instant a father who has leapt from a rock into the whirlpool to die, his small son in his arms, comes up alive and turns back to life; the Meiji essayists made it the Japanese for a conversion, the change of heart that turns a life.
A century of use wore it to everyday size: a fresh start, turning over a new leaf. In Chinese the same two characters mean scheming; the Japanese word turned that into the mind's spring released.
We want you to be as happy with the meaning as you are with Master Takase's art. To learn more, visit Philology of 心機一転.