Wild. Thanks for the keywords to search-engine this with. I found "Reverse engineering the Motorola Sensorhub: Part 1"[0]. To this point I hadn't thought about how they might have implemented the feature. That article sheds some light on it.
replyThat's the one!
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That's the right chip. The other comment shows off the article. I forgot that it was called the "sensor hub", that's why I couldn't find the post showing how it works.
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