The essence is simple: if credit card companies will lend money at 0%, you can borrow it and then save it at a high interest rate so you're earning money on cash they have lent you for free. My fondest stoozing memory is being lent thousands of pounds on Egg's credit card only to shove it into Egg's savings account, so it paid me hundreds of pounds in interest on its own money.

The largest stooze I've heard of was £ 80,000 of debt put into an offset mortgage, netting that stoozer nearly £ 5,000 a year in reduced interest payments. ...

Card providers have introduced balance-transfer fees when debts are shifted to interest-free offers, but cards offering 0% on purchases escape the fees, making them the core equipment for building your "stooze-pot".
—Martin Lewis, "Get £ 1,200 of free money on your plastic," Sunday Times, May 13, 2007

"Stooze" is derived from the nickname of an online financial adviser who developed the practice of taking advantage of the introductory interest-free period offered by credit card companies to borrow money for investing profitably elsewhere.
—Ruth Wajnryb, "The explosion of the English tongue," Sydney Morning Herald, February 24, 2007

But while Kelly has used her cards to avoid interest, some borrowers take the process one step further and claim to make up to pounds 2,000 a year from these introductory deals. Julian Myton, a software engineer, describes himself as a "stoozer" — a consumer who borrows to the hilt on credit cards and invests the money in high-interest savings accounts. ...

He says he has had some difficulty in getting new cards recently, although there was no problem at all for the first couple of years. He adds: "Even now there is always someone prepared to offer an introductory deal. Even if it is not 0 per cent it is still possible to make money from stoozing.
—"'You can avoid paying interest altogether'," Sunday Telegraph, November 7, 2004