It‘s a way to sign transactions without ever connecting your cold wallet with a computer and the internet.
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More info 🙏
Rather than directly connecting the hardware wallet to the computer for transaction signing, you generate the transaction on the computer using a wallet such as Sparrow. Subsequently, you export it to a microSD card, insert the card into your hardware wallet, sign the transaction, and then return the microSD card to the computer for broadcasting.
Consider exploring Coldcard as an example.
I highly recommend nostr:npub1rxysxnjkhrmqd3ey73dp9n5y5yvyzcs64acc9g0k2epcpwwyya4spvhnp8 guide on the Coldcard if you are interested in it:
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