On the iPhone 4S, you can trigger Siri in one of two ways. You can either long press the home button, or you can turn on the “Raise to Speak” option under System Preferences and trigger Siri just by lifting your 4S towards your face.
That might not seem like a big deal, but that little feature led to Apple redesigning the way the iPhone’s IR proximity sensor works in the iPhone 4S.
As the video above shows, on the iPhone 4, the IR proximity light only came on when you made a call, started Skype or launched another app that might need to dim the screen to conserve battery life by shutting down the screen when it is close to your face.
With the iPhone 4S, though, the IR sensor is, in the words of iFixIt, almost neurotic about wondering how close your face is to the device. Not just when you’re on a call… all the time!
As it turns out, it all has to do with Siri’s “Raise to Speak” feature. The IR light has to poll constantly to see if it is being raised to a user’s face, at which point, it will launch Siri.
Christian Zibreg over at 9to5Mac seems to think that this is why Siri can’t run on any pre-iPhone 4S hardware, but I don’t see anything here that suggests this is a hardware limitation: it’s the iPhone 4S operating system that is telling the IR sensor to keep polling. That should be emulable when a proper Siri jailbreak solution comes out. It is, however, a nice example of Apple getting even the small details right.