What role do global reserve currencies (such as the US dollar) play in a crisis?

好的，关于这个问题，我来给你打个比方，让你更好地理解。

想象一下，全球经济是个热闹的大集市，每个国家都有自己的货币（比如人民币、欧元、日元），就像是集市里每个摊主自己印的代金券。你可以在自己摊位或者少数几个接受你代金券的摊位上买东西。

但是，这个集市里有个“硬通货”，大家都认它，它就是美元。你可以用它在几乎任何一个摊位上买东西，尤其是最重要的商品，比如石油、粮食、高科技产品，都只认这个硬通货。这个硬通货，就是我们说的“全球储备货币”。

现在，集市上突然着火了（也就是爆发了金融危机），大家会怎么办？

1. The Safe Haven Effect: Everyone Rushes for the Sturdiest 'Safe'

When a fire breaks out, the various vouchers (other national currencies) you hold might quickly become worthless. Everyone's first reaction is to quickly exchange what they have for the most valuable and safest assets.

The US dollar is that sturdiest safe. Because the United States is the world's largest economy and has the strongest military, people believe it won't easily collapse. So, when a crisis hits, global investors (whether individuals, companies, or nations) will frantically sell off their domestic assets (stocks, real estate, local currency) to convert them into US dollars or buy US Treasury bonds.
This behavior is called a 'flight to safety'. This leads to a strange phenomenon: even if the crisis might have originated in the US (like the 2008 financial tsunami), the dollar paradoxically strengthens because money from all over the world floods into the United States.

In short: During a crisis, holding US dollars is like taking shelter in the strongest fortress during a storm.

2. The 'Blood' of Global Transactions: The More Chaotic, the More It's Needed

Even though the marketplace is on fire, business can't completely stop. The trade of essential goods like oil and grain must continue. And traditionally, these commodity transactions are settled in US dollars.

At this time, all countries urgently need US dollars to import necessary goods. For example, if a country needs to import oil, it must first exchange its own currency for US dollars before it can buy on the international market.
At the same time, many countries and companies have debts denominated in US dollars. When a crisis hits, everyone fears you might not be able to repay, and they might demand early repayment. This further intensifies the demand for US dollars.

In short: The US dollar is the 'settlement currency' for global trade, and during a crisis, demand for it surges instantly, leading to a 'dollar shortage'.

3. The Role of the 'World's Central Bank': Opening the Floodgates to Save the Market

When the whole world lacks US dollars, it's like a person lacking blood; the entire system goes into shock. At this point, the US central bank—the Federal Reserve (The Fed)—has to step in and play the role of the 'world's central bank'.

It does this through a tool called 'Currency Swap Lines', directly 'lending' large amounts of US dollars to the central banks of other countries (primarily close allies), such as the European Central Bank and the Bank of Japan.
These central banks then lend the received US dollars to their domestic banks and businesses, alleviating their urgent need for dollars. This is like a main reservoir providing emergency water supply to various dried-up sub-reservoirs.

In short: The Fed prevents the global financial system from collapsing due to a dollar shortage by 'printing money' and 'lending money' to the world.

In Summary

In a global crisis, the US dollar plays a very contradictory and central role:

On one hand, it's a 'stabilizer' for the crisis. Because the US dollar serves as the ultimate safe haven and the Fed as the ultimate 'liquidity provider', the global financial system doesn't completely collapse; there's a final hope.
On the other hand, it's also an 'amplifier' of the crisis. When everyone rushes to acquire US dollars, other countries' currencies depreciate sharply, and capital flows out frantically, exacerbating the economic woes of these nations. Furthermore, the Fed's decisions (such as raising or lowering interest rates) directly impact the globe, forcing other countries to passively follow suit and losing much of their autonomy.

Therefore, you can view the US dollar as the 'ballast stone' of the global financial ship. You might not notice it during calm seas, but once a fierce storm arrives, everyone relies on it to stabilize the ship, even though it might cause the ship to sway violently, making some weaker passengers seasick or even throwing them overboard.