diff --git a/HACKING.md b/HACKING.md
index 7d59083a612166b5c317c75458e59c0563ffbac8..6ac6dc23e418ac31b6c69080312f73df59903033 100644
--- a/HACKING.md
+++ b/HACKING.md
@@ -24,3 +24,12 @@ Some rules about threading in Briar Desktop:
 
 * Always use local variables instead of the composable State objects inside
   another thread: [Source](https://code.briarproject.org/briar/briar-desktop/-/merge_requests/55#note_57632)
+
+## Testing with multiple app instances
+
+When experimenting with multiple app instances, it can make sense to
+decrease the polling time in briar core so that adding contacts succeeds
+quicker. In `RendezvousConstants`, change
+`long POLLING_INTERVAL_MS = MINUTES.toMillis(1);`
+to something lower such as
+`long POLLING_INTERVAL_MS = SECONDS.toMillis(10);`.