BIP39 describes how to generate a multi-word phrase and then how to convert it to a seed. It states that phrase is directly hashed into a binary seed, so it gives us plausible deniability ("any phrase can work"), but at the same time the phrase contains the checksum, so I can't provide "any" phrase. If I tell some guys another phrase that happens to have a broken checksum, that will easily notice that. Should I understand that "plausible deniability" applies only to a set of all "valid" phrases, i.e. those with valid checksum? Maybe this should be clarified better in the BIP.
First, an initial entropy of ENT bits is generated. A checksum is generated by taking the first (ENT / 32) bits of its SHA256 hash. This checksum is appended to the end of the initial entropy.
Described method also provides plausible deniability, because every passphrase generates a valid seed (and thus deterministic wallet) but only the correct one will make the desired wallet available.
https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki