A Tamahagane Bitcoin card is a "cold storage" Bitcoin wallet. The secret phrase which provides access to the wallet is engraved on it and hidden by a tamper-evident covering. It also has a public Bitcoin address to receive funds.
The wallet is generated on a system that's completely and permanently offline, never connected to the Internet. The seed phrase is then engraved on the card and immediately concealed by the tamper-evident covering, and the system is immediately reset with all data securely wiped each time.
The tamper-evident sticker that covers the secret phrase cannot be removed and replaced without visible evidence, and its unique code verifies that it has not been replaced by a new one. This guarantees that the secret phrase has not been seen by anyone except the recipient.