mercurial/help/extensions.txt

Mercurial has the ability to add new features through the use of
extensions. Extensions may add new commands, add options to
existing commands, change the default behavior of commands, or
implement hooks.

Extensions are not loaded by default for a variety of reasons:
they can increase startup overhead; they may be meant for advanced
usage only; they may provide potentially dangerous abilities (such
as letting you destroy or modify history); they might not be ready
for prime time; or they may alter some usual behaviors of stock
Mercurial. It is thus up to the user to activate extensions as
needed.

To enable the "foo" extension, either shipped with Mercurial or in
the Python search path, create an entry for it in your hgrc, like
this::

  [extensions]
  foo =

You may also specify the full path to an extension::

  [extensions]
  myfeature = ~/.hgext/myfeature.py

To explicitly disable an extension enabled in an hgrc of broader
scope, prepend its path with !::

  [extensions]
  # disabling extension bar residing in /path/to/extension/bar.py
  bar = !/path/to/extension/bar.py
  # ditto, but no path was supplied for extension baz
  baz = !