The new Apple iPhone 6s' Siri does not listen to voice commands when it is placed face down or if inside a pocket. All these because of how the M9 coprocessor works.

Apple's flagship phones this year are powered by an A9 CPU which works with an integrated M9 coprocessor, that is is utilized for "Hey Siri" input and listen to voice commands, almost all the time, said Apple Insider.

According to the iPhone 6s website, the M9 motion coprocessor integrated into the A9 chip aims to have the user do more for longer periods with great battery life and performance.

Connected to the accelerometer, barometer, compass and gyroscope, it provides various range of fitness tracking. iPhone 6s can measure the user' walking and running pace, not just distance, elevation changes and steps.

With regard to the Apple iPhone 6s' Siri, M9 intelligently waits for voice commands. It is because of M9 that users can easily activate the digital assistant with "Hey Siri" to a nearby iPhone 6s. Previously, the motion coprocessor of Apple's mobile devices was used to constantly track activity and steps. In this year's A9 CPU, the integrated M9 can listen to voice as well.

Previous iPhones get battery drained with "Hey Siri," hence, the need for an external power. Though M9 listens to "Hey Siri" all the time basically, there are times that it does not. Siri does not work when the iPhone 6s is lying face down, or is inside a pocket.

With iOS 9, Apple uses the proximity sensor to prevent the display from turning on when the handset is within a pocket or lying face-down. Being the most power-hungry iPhone component, the screen is kept dark when the user cannot see it, like when it is lying face down. It is a practical way to extend battery life.

It is practically unlikely for the user to command the Apple iPhone 6s' Siri in his pocket when the speaker and microphone cannot work well. Therefore, users who want to give voice commands with "Hey Siri" should ensure the handset is on a table, face up, and not inside a pocket.