Crib dragging
Sliding a guessed piece of plaintext along the ciphertext and watching where it breaks the cipher's rules.
A crib is a guessed or known piece of plaintext, such as a weather phrase or a standard greeting that operators tended to send at the start of a message. Crib dragging means placing the crib at every position in the ciphertext and checking whether that placement is consistent with the cipher.
With Enigma the rule was simple: no letter could ever be encrypted to itself. Any position where the crib produced such a clash could be discarded. The method was mechanical, and it suited Alan Turing's bombe, which tested rotor settings instead of working them out by hand. Clearing out the false positions was often what made the rest of the work possible.