"China has opened the door to imports of rice from the United States for the first time ever in what analysts took to signal a warming of relations between the world's two biggest economies after a frosty year marked by tensions and tit-for-tat tariffs."
What does the word "in what" refer to?
The wording "in what" is not a simple unit in this sentence.
The model sentence includes the opinion of unidentified analysts. Let's look at a version of this sentence that ignores the analysts and simply presents their opinion as fact:
China has opened the door to imports of rice from the United States for the first time ever in a warming of relations between the world's two biggest economies after a frosty year marked by tensions and tit-for-tat tariffs.
On second thought, this version goes a step too far. It's not the analysts presented opinion that relations have warmed. Rather, the opinion is that this event shows that relations will (or may) become warmer.
China has opened the door to imports of rice from the United States for the first time ever in [an event that signals] a warming of relations between the world's two biggest economies after a frosty year marked by tensions and tit-for-tat tariffs.
From here, it's easy enough to explicitly mark that this is opinion rather than fact:
China has opened the door to imports of rice from the United States for the first time ever in [an event taken to signal] a warming of relations between the world's two biggest economies after a frosty year marked by tensions and tit-for-tat tariffs.
And, finally, we can introduce who it is that holds this opinion, as well as placing this statement of opinion in the past tense:
China has opened the door to imports of rice from the United States for the first time ever in [what analysts took to signal] a warming of relations between the world's two biggest economies after a frosty year marked by tensions and tit-for-tat tariffs.
Analysts took something to signal a warming of relations. The phrasing "what analysts took to signal a warming of relations. . ." represents that something. Whatever that something is (ideas like event, change, and circumstances all seem to fit) is the object of the preposition "in".
In the sentence, in what refers to the subject just discussed.
In other words, in what refers back to China has opened the door to imports of rice from the United States for the first time ever.
The sentence can be rephrased in a way that will express something essentially the same, or the same enough to convey the idea:
China has opened the door to imports of rice from the United States for the first time ever, which analysts took to signal a warming of relations between the world's two biggest economies after a frosty year marked by tensions and tit-for-tat tariffs.
You can also consider the following:
What did she do? She ran back into the burning house.
What would you call that? A moment of heroism.She ran back into the burning house in what was a moment of heroism.
She ran back into the house. (Her doing that) was a moment of heroism.
