Everyone that owns an iPhone 4S or has at least played with its voice assistant Siri knows that there are two ways to activate her. You can hold down to home button and she will pop right up, ready to assist. Or, you can simply hold the phone up you your head and Siri will come alive.

She knows that you want to talk.

Teardown blog iFixit has uncovered the reason why this works during a teardown of the new Apple device, and it involves nearly constant work from your phone’s infrared sensor.

The presence of an infrared sensor in an iPhone isn’t new – they’ve been there for years. In the past, the infrared sensor would turn on when a user made a call. The sensor is a way for the phone to determine how close it is to your face – so it would activate in order to shut down the screen. Your iPhone is nice enough to not want you to hit buttons with your cheek during a call.

But when they tore down the 4S, they realized that a new component makes it so that the IR sensor is constantly wondering where it is in relation to your face, anytime that the screen is activated. And that’s how Siri’s secondary retrieval mechanism functions.

Siri is ready and waiting to answer her master’s beck and call at any time. And in order to be as attentive as a personal assistant ought to be, Apple had to design the proximity sensor to be as vigilant as Big Brother, but as cute as Little Sister. So whenever the screen is active, the proximity sensor is active too. Thus, whenever you raise the iPhone 4S to your face, Siri is ready to take orders.

Of course, this immediately prompts one to wonder about its effect on battery life – an issue that has plagued the iPhone 4S since its launch. Just how much drain could that constant infrared activity have on your phone? I guess you could disable the “raise to speak” functionality and see for yourself.

In other Siri news, Apple has quelled recent rumors that they might be bringing it to older iPhones.