The Enigma machine worked reciprocally so that an identical machine with identical settings would, if fed the enciphered letters, show the deciphered letters on the lampboard. Hut 6 had Enigma replica machines that were logically identical to the machines that the Germans were using.
To decipher the intercepted messages required that the selection of rotors, the ring settings and the plugboard connections were known. At this time, the first three letters of the prelude to the message were used as an indicator to tell the receiving operator the letters that should appear in the windows for this particular message.