diff --git a/artiq-fast/windows/README.md b/artiq-fast/windows/README.md
index 07962a7..9272098 100644
--- a/artiq-fast/windows/README.md
+++ b/artiq-fast/windows/README.md
@@ -18,3 +18,9 @@ nix-build build.nix
 ls -la ./result
 ```
 Results in a symlink to the image in the nix store
+
+
+# Impure/pure mode
+Sometimes it can be useful to build the image _outside_ of the Nix sandbox for debugging purposes.
+
+For this purpose we have an attribute called `impureMode` which outputs the shell script used by Nix inside the sandbox to build the image.