In an [old cryptography FAQ](http://doc.rz.ifi.lmu.de/FAQ/cryptography-faq/part08/index.html), I found the following step described for determining a the length of the key a cipher was repeatedly XORed against:

> 
> 1. Discover the length of the key by counting coincidences.
>  (See Gaines [GAI44], Sinkov [SIN66].) **Trying each displacement of
>  the ciphertext against itself,** count those bytes which are equal. 
>  If the two ciphertext portions have used the same key, something
>  over 6% of the bytes will be equal. If they have used different
>  keys, then less than 0.4% will be equal (assuming random 8-bit bytes
>  of key covering normal ASCII text). The smallest displacement which
>  indicates an equal key is the length of the repeated key.
> 
> 
> 

The wording of the bolded text is the part I'm confused with. Given that my plaintext equals the string "This is a secret message", would I be comparing the first n number of bytes with a second set of n number of bytes that is offset from the first (e.g. the xor'd version of "Th" with "is", if I was comparing 2-byte chunks of the cipher against itself)?

If not, what's the correct way to interpret this passage? Thank you.