It is the service that does it, and the button just calls the service,
If it fails, you get a persistent notification.
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Interesting, I had no idea. Gonna test this out on my dev instance and then update the docs. Thanks!
https://github.com/home-assistant/core/blob/a2651845f379992231fd7b9c8458828036296ee0/homeassistant/components/homeassistant/__init__.py#L101-L122 The code at least say it does 
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Strangely enough, it does exactly what the code says! 
Created a PR to update the doc with this info. Thanks for the heads up!
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How strange!! 