Wasn't it that they could feed in certain known possible letters to cut down on the number of options? Something like, "this encrypted segment is usually the weather so the s is probably the word 'sunny'".
https://www.youtube.com/watch?v=9HH-asvLAj4
Here's a really good video I've watched before and since forgotten the details of which explains Colossus and its context
Earlier in the war the Germans typically sent their call signs in plaintext. Many of the intercept operators in the “Y stations” in the UK became familiar with the “hands” of various German operators. Later in the war the Germans started encrypting their call signs, but Bletchley was generally able to distinguish them by the intercept operator’s familiarity with the “hands” of the German Morse operators.