BOAZ WEINSTEIN didn’t know it, but he had just hooked the London Whale.


It was last November, and Mr. Weinstein, a wunderkind of the New York hedge fund world, had spied something strange across the Atlantic. In an obscure corner of the financial markets, prices seemed out of whack. It didn’t make sense.


Mr. Weinstein pounced.


As the financial world now knows, what was out of whack was JPMorgan Chase & Company. One its traders, Bruno Iksil, the man later nicknamed the London Whale for his outsize trades, was about to blow a multibillion-dollar hole in the mighty House of Morgan.


But the resulting uproar, in Washington and on Wall Street, has largely obscured a simple truth of the marketplace. Yes, Morgan lost big — but, as Mitt Romney has pointed out, someone else won. And that someone or, rather, those someones, turn out to be Boaz Weinstein and a wolf pack of like-minded hedge fund managers.


In the London Whale, these traders saw a rich opportunity, and they seized it with both hands. That, after all, is the way hedge funds roll. His cool calculus has made Mr. Weinstein a very rich man: he is in talks to buy the Fifth Avenue co-op of a reclusive heiress, Huguette Clark, for $24 million.


It might seem remarkable that someone like Mr. Weinstein, a man virtually unknown outside of financial circles, could deal such a stinging blow to one of the world’s largest, most respected banks. Jamie Dimon, the chairman and chief executive of JPMorgan and a face of the banking establishment, is struggling to contain the damage from what he has called a “terrible, egregious mistake.” The loss — JPMorgan put it at $2 billion, but it may turn out to be $3 billion or more — has renewed calls for stronger financial regulation.


Given the secretive nature of the business, few on Wall Street, including Mr. Weinstein, were willing to speak publicly about how the hedge funds harpooned the London Whale. But interviews with more than a dozen hedge fund managers, investors and traders pull back the curtain on the ways of this band of traders, and on what really happened.


One thing is sure: Mr. Weinstein, 38, played a central role in this, one of the biggest trading blowups since the financial crisis of 2008. Mr. Iksil and his colleagues in the chief investment office at JPMorgan may have lighted the fire, but Mr. Weinstein and his cohorts fanned the flames. In the hedge fund game, a business in which ruthlessness is prized and money is the ultimate measure, Mr. Weinstein is what is known as a “monster” — an aggressive trader with a preternatural appetite for risk and a take-no-prisoners style. He is a chess master, as well as a high-roller on the velvet-topped tables of Las Vegas. He has been banned from the Bellagio for counting cards.


From offices on the 58th floor of the Chrysler Building in Midtown Manhattan, Mr. Weinstein runs a $5.5 billion hedge fund firm called Saba Capital Management. (“Saba” is Hebrew for “grandfatherly wisdom,” a nod to his Israeli roots.) It was there, last autumn, that he noticed an aberration in the market for credit derivatives. He knew from experience what it was like to lose a lot of money at a big bank. Before starting Saba, he was responsible for a team that lost nearly $2 billion, in the depths of the financial crisis, at Deutsche Bank. Others lost even more. Last November, however, he saw that a certain index seemed to be trading out of line with the market it was supposed to track. He and his team pored through reams of data, trying to make sense of it.


Finally, as Mr. Iksil, the London Whale, kept selling, Mr. Weinstein began buying.


At the time, traders in London had no real idea that JPMorgan was behind the trades that were skewing the market in credit derivatives. In fact, they weren’t even sure that it was a single bank or trader. But soon the City of London, Europe’s financial hub, was buzzing. Whoever the mysterious trader was, he or she kept selling derivatives intended to rise in value in the event that certain corporate bonds became riskier. The volume of trades was off the charts. Who could possibly sell so much? And, what if the trade reversed, as it inevitably would?


And so the battle lines were drawn. On one side was JPMorgan, the American banking giant that had weathered the financial crisis far better than so many of its peers. On the other were hedge fund managers, including Mr. Weinstein at Saba.


Such standoffs are not uncommon on Wall Street. An aggressive trader makes a wrongheaded bet, then doubles down to scare off competitors on the other side of the trade. Market rivals often get slapped down, unwilling to keep buying as the other side is selling, or vice versa. For traders with the backing of a major bank, like JPMorgan, the task is much easier.


But not always. Sometimes, the other side sits tight, then hits back in force. And it does so in numbers.