The phrase "stunning reversal" means a sudden and surprising change of a situation or outcome that is extremely impressive or shocking. Full definition
A question emerges: Did government intervention trigger Apple's stunning reversal of fortune, or was it bound to happen anyway? (businessinsider.com)
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It was a fairly stunning reversal for Apple, and one that came without any PR campaigns or orchestrated news releases — just a series of tweets from a senior executive, effectively apologizing for the company's prior stance: (fortune.com)