The idea is that if you force the browser to produce the ciphertext of a block like "kie: sessionid=f", where the first 15 characters are already known, then yo

The idea is that if you force the browser to produce the ciphertext of a block like "kie: sessionid=f", where the first 15 characters are already known, then you don't have to try many candidate plaintext blocks through your adaptive-chosen-plaintext oracle to find that unknown 16th character.