The idea is that a digital key to decrypt and recover sensitive or personal data should have long-term durability and not be stored on any computer or network. The length of secure cryptographic keys restricts memorization, so the secret key takes the form of a 2D barcode, a machine-readable print. Early implementations of a paper key by the company Safeberg use a Data Matrix barcode. Safeberg Trusted paper key - 4.096 bits RSA key in a Datamatrix barcode or human-readable base 16 digits. "Paperkey - an OpenPGP key archiver" by David Shaw
The user stores the printed key in a secure location. To avoid abuse, the key can only be used in combination with a ‘normal’ password.
The user can extract the key by creating a digital photo or scan of their paper key and feeding it to cryptographic software that extracts the key to decrypt the data.