According to Christopher Elias in “MF Global and the great Wall St re-hypothecation scandal” (via Zero Hedge)
A simple example of a hypothecation is a mortgage, in which a borrower legally owns the home, but the bank holds a right to take possession of the property if the borrower should default.
In investment banking, assets deposited with a broker will be hypothecated such that a broker may sell securities if an investor fails to keep up credit payments or if the securities drop in value and the investor fails to respond to a margin call (a request for more capital).
Re-hypothecation occurs when a bank or broker re-uses collateral posted by clients, such as hedge funds, to back the broker’s own trades and borrowings. The practice of re-hypothecation runs into the trillions of dollars and is perfectly legal.
Re-hypothecation transactions are off-balance sheet and are therefore unrestricted by balance sheet controls. Whereas on balance sheet transactions necessitate only appearing as an asset/liability on one bank’s balance sheet and not another, off-balance sheet transactions can, and frequently do, appear on multiple banks’ financial statements. What this creates is chains of counterparty risk, where multiple re-hypothecation borrowers use the same collateral over and over again. Essentially, it is a chain of debt obligations that is only as strong as its weakest link.
With collateral being re-hypothecated to a factor of four (according to IMF estimates), the actual capital backing banks re-hypothecation transactions may be as little as 25%. This churning of collateral means that re-hypothecation transactions have been creating enormous amounts of liquidity, much of which has no real asset backing.