Scientists call it “tickling the dragon’s tail," Custer said, because the experiment approaches but stays below the stage at which the fission of nuclear materials sustains an ongoing series of chain reactions.
According to https://en.wikipedia.org/wiki/Demon_core Feynman was the person behind that expression.
Scientists referred to this flirting with the possibility of a nuclear chain reaction as "tickling the dragon's tail", based on a remark by physicist Richard Feynman, who compared the experiments to "tickling the tail of a sleeping dragon".