U.S. Treasury Secretary Steve Mnuchin surprised Wall Street by arranging a phone call on Monday involving the so-called President's Working Group on Financial Markets. But if the stated purpose was "to assure normal market operations," for investors the move seemed to deepen their anxiety amid a deepening slump in stocks, ongoing government shutdown, and growing tension between the White House and Federal Reserve.

But just what is the working group, known as the PWG and more informally as the "Plunge Protection Team?"

Raising more questions

Formed after the "Black Monday" stock market crash in 1987, the PWG is designed to connect Wall Street and Washington on financial market issues. The group is led by the Treasury Secretary and includes the Fed chief, Chair of the Securities and Exchange Commission, and Chair of the Commodity Futures Trading Commission.

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The PWG doesn't hold formal power. Rather, it's designed to make sure key market participants are communicating about critical issues. One reason why Mnuchin's move may have caused a measure of alarm — the last time the Plunge Protection Team played a prominent role was during the financial crisis a decade ago.

Mnuchin's appeal to the PWG also came as he sought to reassure financial markets by saying he'd spoken this weekend with the CEOs of six big U.S. banks -- a move that analysts said seemed to backfire. The Dow lost more than 650 points Monday, while the S&P 500 and Nasdaq also continued their descent after tumbling last week.

"These actions seem to raise more questions than answers -- especially as no one had seemed to raise any concerns related to these issues," Raymond James analyst Ed Mills said in a note to clients.

Are we missing something?

So what are analysts really worried about? That the government and top bank executives may know something other investors don't. Such concerns are flaring as stocks, which only a few months ago were setting record highs, beat a hasty retreat and are on track for their worst December since the 1930s.

The mood on Wall Street contrasts with the normal pattern around the holidays, when optimistic investors often push stocks higher in a "Santa Claus rally."

"This is the type of announcement that raises the question of whether Treasury sees problems that the rest of the market is missing," said Jaret Seiberg, an analyst with Cowen Washington Research Group, in a note. "Not only did [Mnuchin] consult with the biggest banks, but he is talking to all of the financial regulators on Christmas Eve. We do not see this type of announcement as constructive and worry that it can trigger the vary panic that Treasury wants to avoid."

As Quartz notes, conspiracy theories about the PWG also tend to emerge during times of big market movements. That may be one reason experts question the need to scramble group members.

"The working group is a way for a president to say HELLO AMERICA I AM DOING A THING," Financial Times columnist Brendan Greeley wrote, noting that functionally it's been replaced by the Financial Stability Oversight Council, a group of federal and state regulators created under the 2010 Dodd-Frank financial reform law.

"Mr. Mnuchin could just as easily call on the order of the garter to take up arms and defend the realm. He's throwing glitter on a football. This isn't how any of this is done."