In a repo transaction one institution (the lender) agrees to buy an asset from another institution (the borrower) and sell the asset back to the borrower at a pre-agreed price on a pre-agreed future date (a day, a week or more). The lender takes a fee (repo interest rate payment) for ‘buying’ the asset in question and can sell the asset in the case that the borrower does not live up to the promise to repurchase it. The fundamental purpose of this circular transaction is to lend and borrow funds (and, in some cases, securities). While financial institutions use it to raise finance, central banks use it in monetary policy.
The very same technique Lehman Brothers used and perfected through the now infamous Repo 105. Read further for the NPR account :
Everybody knows the bank isn’t really selling the bond to the big company — it’s really borrowing money. So under accounting rules, the assets a bank uses in repo deals stay on the bank’s balance sheet.
But when Lehman Brothers wanted to make it look like it wasn’t borrowing so much money, the company used a special technique to get around this rule. It did repo deals where it took slightly less cash than the asset was worth.