Ann: Business Operations Update, page-110

Like @chris1983 says and to make it simple, you buy a TV for $2000, SPT/ the merchant put a security hold on your cc for the full $2000 (so you cant go double spend) and then they take the first payment ($500) which gives you say 30 days to pay down the $500 (no interest) then the next $500 is taken and so on.

The current way is you buy a TV for $2000 and the interest is accrued on the full outstanding $2000 from the 30 day mark.