auto-compression-mode is an interactive compiled Lisp function in `jka-cmpr-hook.el'.

(auto-compression-mode &optional ARG)

Toggle Auto Compression mode.
With a prefix argument ARG, enable Auto Compression mode if ARG
is positive, and disable it otherwise. If called from Lisp,
enable the mode if ARG is omitted or nil.

Auto Compression mode is a global minor mode. When enabled,
compressed files are automatically uncompressed for reading, and
compressed when writing.