In a nutshell

Here's Brad Setser:

Implicitly, Geithner and his colleagues seem to have concluded that the
“great unwind” has limited the private sector’s ability to absorb the
banks troubled assets. Key players no longer can borrow the funds
needed to make large bets on troubled mortgage-backed securities. By
providing credit to those willing to buy bad assets, the US government
hopes to push up their market price up, and in the process induce the
banks now holding these assets to sell. The US government in effect is
providing the financial system with leverage to facilitate – one hopes
– a transition to a less leveraged financial system. The amount that
private investors have to put down – relative to the amount they are
spending – is a key detail.

The post is interesting throughout.