diff --git a/CONTRIBUTING.rst b/CONTRIBUTING.rst
index 01dd7e392320b3d26e255996e00326519ebbb8ee..da0e50b9f9748e71696c5d8cf02181c470d6c467 100644
--- a/CONTRIBUTING.rst
+++ b/CONTRIBUTING.rst
@@ -50,13 +50,18 @@ To update to the current master@HEAD revision, run:
    nix-shell --run 'niv update tahoe-lafs-master --branch master'
 
 It is also possible to pass ``pull/<pr-number>/head`` to test against a specific PR.
-When feasible we also test against a released version of Tahoe-LAFS.
+
+The Nix package depends on a version of Tahoe-LAFS determined by the "tahoe-lafs" niv entry.
+When feasible this is a released version of Tahoe-LAFS.
 To update to a new release, run:
 
 .. code:: shell
 
    nix-shell --run 'niv update --rev tahoe-lafs-A.B.C tahoe-lafs'
 
+When it is not feasible to use a released version of Tahoe-LAFS,
+niv's ``--branch`` or ``--rev`` features can be used to update this dependency.
+
 .. note::
 
    Since tahoe-lafs doesn't have correct version information when installed from a github archive,