diff --git a/docs/impl/plans/INITIAL.md b/docs/impl/plans/INITIAL.md
index b15b0ce..72e7045 100644
--- a/docs/impl/plans/INITIAL.md
+++ b/docs/impl/plans/INITIAL.md
@@ -54,3 +54,16 @@ app is developed with no Jetson present. The stream carries a flag when it does.
 Rendering synthetic readings indistinguishable from real ones, on a screen in a
 moving car, is a safety problem. The indication belongs on the display surface
 itself, not in a settings page or a startup log.
+
+## This app is the audio sink
+
+The tablet owns the car's aux input and is the music player, so the Orin's speech
+has to arrive here to be heard.
+
+- TTS arrives as **discrete utterances** over the reliable channel, not as a
+  real-time stream. Receive the whole thing, then play it — no jitter buffer.
+- Duck other audio with `AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK` for the duration of
+  the utterance, then release. That is what makes Spotify drop under the voice
+  without either side coordinating.
+- The microphone stays on the Orin. This app never captures audio; ASR input never
+  crosses the boundary.