yEnc is a transfer encoding used for attaching files to Usenet newsgroup messages (and, rarely, e-mail and other kinds of messages). It is a more size-efficient alternative to Uuencoding.
It is justly characterized as "quick and dirty", requiring crude Uuencoding-style heuristics to detect it, instead of utilizing the robust-but-complex MIME standard that predates it.
A yEnc-encoded file is delimited by lines that begin with "
=ybegin" and "