The way it works is the card gets linked to an account on signup. This account is a non-interest bearing card account. When you deposit funds, we place those funds in other accounts. The other accounts are determined by what products we have available from our partner banks, and your historic usage patterns.
For example, if you start spending less, we'll move more of your money into an interest bearing account so that you can benefit from your financial restraint.
As you spend money, we (in real time) move funds to the card account from your other accounts to cover the cost of anything you buy using the card.
As our portfolio of financial products grow, we'll continually rebalance your money across these products to maximize your return.
This system automates what most people try to achieve, but banks make difficult to do: namely, earn as much interest on money you have and pay as little interest on money you borrow.
We do this because it is the right thing to do for our customers & quite easy for a computer to manage. Other banks don't do this because they earn significant revenue when people make mistakes in managing their money. We have none of those punitive fees, so our interests are aligned with our users.
(PS: I'm in London right now. What's up with English Bacon??)