http://tzoz3bensgxyzs7da7lpgsn3a74h7hlbm4wa6ytq2tg6ktd57w22vqqd.onion/doc/about/openpgp_keys/signing_key_revocation/index.en.html
These cryptographic shares can be
put together to reassemble the revocation certificate and revoke the
Tails signing key. This may be needed in case something really bad
happens to us and we are not able to do the revocation ourselves.
Note: In all this document, 'us' refers to the Board.
You can read a complete description of the distribution mechanism on:
https://tails.net/doc/about/openpgp_keys/signing_key_revocation.