[nijave]

It's effectively just multiple key protectors. TPM+PIN is one way to protect the data encryption key. You can also backup the actual encryption key (which is the recovery key). You can also add a password that protects the key or back the key up to an online Microsoft account or enterprise Active Directory account.

[nutto]

The actual encryption key for the volume data isn't the recovery key, it's the FVEK (full volume encryption key), which is encrypted using the VMK (volume master key).

The recovery key is a 128-bit value (entered as 8 groups of 7 digits, each of which when divided by 11 gives a 16-bit value, where a non-zero remainder indicates the group has been incorrectly entered) which gets hashed repeatedly, with a salt, to derive a 256-bit key that decrypts a copy of the VMK.