As far as I’m aware this works with curve because they lend you the money and pre authorise all you cards and the other charges drop for the cards you don’t want to use
I could be wrong but I don’t think Curve lends money, my view of Curve is simpler.
It authorises the card being used at time of purchase like an online transaction (as it essentially is). If you time machine, it just authorises the other card and cancels the authorisation on the first card and/or processes a refund for it.
There must be some Curve reauthorisation/refund sleight of hand, because you’re able to use Go Back in Time up to 90 days after a transaction – long after you’d expect it to have settled.