I decided to trade a few ADA for BTC at Bittrex to check this feature out; I traded 4080 ADA for BTC at the rate of 465 satoshis/ADA, and this is how it all went:


It all went smoothly. I traded without any trading fee (due to adjustment of these credits).
At the rate of $8100 per BTC, 0.00004743 BTC equals 38 cents. So it seems like 1 credit = 1 cent.