We continue to find new details about iOS 9’s battery saving Facedown Detection feature. We’ve already covered how it works, and why it is not available on older iPhones, iPads and iPod touches.

As you probably know, the new iPhones 6s and iPhone 6s Plus come with the cool always-on “Hey Siri” feature, so you can use Siri without connecting your device to a power source.

However, there is one exception. The always-on “Hey Siri” feature is disabled by iOS 9’s Facedown Detection feature in the iPhone 6s and iPhone 6s Plus.

Tony, one of our readers who tipped us about it, isn’t impressed. But I think it makes sense to disable the feature, as Siri isn’t very useful when you can’t see the information displayed on the screen. I’m not sure if Apple disabled the feature because of this reason, or to preserve battery life.

So in case, you’re wondering why “Hey Siri” isn’t working on your iPhone 6s or iPhone 6s Plus, just make sure your device is not facedown. The other way to look at it is, this is the only way to disable the always-on “Hey Siri” feature temporarily.

Thanks Tony for the tip!