In cryptography, Russian copulation is a method of rearranging plaintext before encryption so as to conceal stereotyped headers, salutations, introductions, endings, signatures, etc.
This puts all endings and beginnings (presumably the location of most boilerplate phrases) "somewhere in the middle" of the version of the plaintext that is actually encrypted.
For some messages, mostly those not in a human language (e.g., images or tabular data), the decrypted version of the plaintext will present problems when reversing the inversion.
For messages expressed in ordinary language, there is sufficient redundancy that the inversion can almost always be reversed by a human immediately on inspection.
[1] The English phrase suggests that it originally came from an observation about Russian cryptographic practice.