Ordinary traffic crosses the open internet in the clear. Anyone sitting on the path can read what is inside, and everything looks like everything else.

Encryption protects what the message says. It does not hide that the message exists. An encrypted payload on an open channel is an armoured van on a public street: nobody can see the cargo, and everybody can see that something worth guarding just drove past.

NBC encodes the encrypted payload into an image of visual noise, then hides that image inside ordinary content and sends it unencrypted. The armoured van is now just another car in traffic, and nothing on the wire asks to be inspected.

The key holder pulls the noise image back out of the carrier and decrypts it. Without the key there is nothing to find, because on the wire there was never anything but an ordinary picture.