I just watched the movie "The imitation game" (2014) which is based on Alan Turing's biography.
At some point in the movie the machine built by Turing wasn't fast enough to decrypt the Germans' messages, so they had the idea to search for the Enigma setting that deciphers a secret message into a plaintext that contains a known phrase (it was "Heil Hitler" in the movie but I have read that Turing actually searched for the word "eins" in the plaintext). As far as I can understand this is a form of KPA (Known Plaintext Attack).
In the movie Turing says something like "What if we don't have to search through all the possible combinations? What if we only search through the ones that produce a word we know will be in the message?".
I don't understand what it does mean for a machine to "search through all the possible combinations". i.e. I don't understand what was the machine doing before Turing and their teams "upgraded" it to search for "Heil Hitler" in the decrypted plaintext. Suppose they had enough time to let the machine run until it searched through all the possible combinations: how could it know which one was correct? I know that in principle a human could read all the possible generated plaintexts, but that would be unfeasible in practice because it requires a lot of time, so I suppose that's not what was happening. What else, then?