Britian's secret weapon in the war against Hitler

This is it.

Calvocoressi points out that next to the decodes from Hut 6, the most important tool in Hut 3 was the indexing system. His Air Index, of which he is justly proud, was in a large room just off the watch room. On each shift of the watch a team of indexers would take each decoded message and underline key words and to be put on cards-- names of people, places, units, weapons, code words, scientific terms, and such special subjects as oil. It was a huge job, and the Air Index grew to many thousands of cards, so precious that they were photographed and duplicates stored away in another location in case Bletchley should be bombed. Over and over again, reference to the index would be essential to proper interpretation of a decoded message.

Hut Six Story

Breaking the Nazi codes was a vital step in the intelligence war, but it was only a first step. Secrets are more often hidden by fragmentation than by encryption.

[There are] no secret documents in the romantic sense of the words. On any important subject, there is no single document or even group of documents that contain "the secret." No spy could know enough to spot such a document if it existed, and no vacuum cleaner approach to espionage, even should it gather up two or three documents of the highest importance, would lead without all the analytical skills of the humanists to any valid conclusions. Documents do not speak:

Cloak and Gown

Meticulous extraction of figures, names, dates and so on from the multitude of incoming signals meant that the Index contained not only the most minute and accurate details about the enemy's order of battle, developing plans and production states, and personalities high and low in the German commands, but also records of points which, when they first cropped up in a signal, might have seemed trivial and meaningless; later however, when they appeared in another intercept the item already noted on a card in the Index could help to throw a sudden light on some secret, unsuspected weapon or the hidden deployment of a German unit

Ultra Goes to War

A picture of the combat effectiveness of German divisions could be built up by painstaking study of their supply returns, so the likelihood of an attack by corps or army had often to be inferred from separate scraps of information suggesting that several units were assembling in the same area or that a large number of aircraft was under orders to support ground troops at a certain time and place.

ULTRA in the West

This video has a good demonstration by a former Bletchley Park officer Peter Calvocoressi who shows how decrypts and the Index were used together to produce intelligence. It is around the 27.00 minute mark.