The record
With the iPhone 6s and 6s Plus, Apple tied a Siri change to new hardware. The release said the M9 coprocessor built into the A9 chip would keep Hey Siri available at low power, so the phone no longer had to be charging to hear it. Apple also pointed to improved Siri features coming with iOS 9. This entry records an announcement, not independent testing.
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Apple’s announcement, recorded as a company statement. An announcement is not proof of availability or independent verification.