Andon is a signal — a light, a button, a board — that lets whoever is doing the work stop it and call for help the moment something is wrong.
The hardware is incidental. What makes it function is the authority behind it: an operator can halt production. Without that, the light is a notification and the line keeps running with a known defect travelling down it, getting more expensive at every step it passes.
The response is the system
A pull has to bring help in seconds. If what arrives is a supervisor asking why the line is stopped, people stop pulling, and an andon nobody pulls is worse than none at all — it produces a wall of green while problems accumulate underneath.
This makes pull frequency a treacherous metric. It falling could mean fewer problems or less reporting, and the chart looks identical either way.
The principle carries outside manufacturing to any process where a problem can be quietly passed on: a support case flagged rather than escalated in silence, a failing build that stops the pipeline. What transfers is permission to stop, not the light.
