I had to issue an OTP card so that I can change the maximum amount of bank transfer in the banking app. To strengthen the security of the authentication process, they used to be a card with many printed numbers but many Korean financial institutes have been switching to One Time Password (OTP) card.
The size and thickness are exactly the same as the credit card but it contains an e-ink display, mechanical switch and a battery which should last for a couple of years. This guy disassembled the card.
I am not sure exactly how it works but probably it depends on the time – there should be an assumption that the times of the server in the bank and the card are almost synchronized. The guy on the above link said the card didn’t work after 2.5 years probably because the time is not synched well.