What’s going on? Mr. Geithner described a vicious circle in which banks and other market players who took on too much risk are all trying to get out of unsafe investments at the same time, causing “significant collateral damage to market functioning.”

A report released last Friday by JPMorgan Chase was even blunter. It described what’s happening as a “systemic margin call,” in which the whole financial system is facing demands to come up with cash it doesn’t have. (A financial joke making the rounds, via the blog Calculated Risk: “Who is this guy Margin that keeps calling me?”)

The Fed’s latest plan to break this vicious circle is  as the financial Web site interfluidity.com cruelly but accurately describes it  to turn itself into Wall Street’s pawnbroker. Banks that might have raised cash by selling assets will be encouraged, instead, to borrow money from the Fed, using the assets as collateral. In a worst-case scenario, the Federal Reserve would find itself owning around $200 billion worth of mortgage-backed securities.

Some observers worry that the Fed is taking over the banks’ financial risk. But what worries me more is that the move seems trivial compared with the size of the problem: $200 billion may sound like a lot of money, but when you compare it with the size of the markets that are melting down  there are $11 trillion in U.S. mortgages outstanding  it’s a drop in the bucket.

The only way the Fed’s action could work is through the slap-in-the-face effect: by creating a pause in the selling frenzy, the Fed could give hysterical markets a chance to regain their sense of perspective. And to be fair, that has worked in the past.

But slap-in-the-face only works if the market’s problems are mainly a matter of psychology. And given that the Fed has already slapped the market in the face twice, only to see the financial crisis come roaring back, that’s hard to believe.