One of the more interesting features of the Lumia Denim and Windows Phone 8.1.1 updates for high-end devices is 'Hey Cortana'. The new feature was announced back in Berlin during the September Microsoft press conference for the Lumia 830 and Lumia 730.
Passive-voice listening technology found in the Qualcomm 800 and higher chipsets lets users call up the Cortana personal assistant by just using their voice, even when the device is in standby. It is a clever feature that Google first used with their 'Ok, Google' feature on the Motorola X series of smartphones.
Overall, it is a very useful feature for using Cortana truly in a hands-free environment. So when at the computer or while the phone is on your desk, you can ask Cortana to call people, remind if you to do things, ask questions, and more.
Users do need to say 'Hey, Cortana' and wait until the OS wakes up. You can then continue your query when you hear the Cortana chirp. However, you cannot just say in one fell swoop 'Hey, Cortana remind me to get some bread at the store'. At least not yet, although Microsoft is likely toying with various iterations of this service.
Training is simple enough as users are walked through the process when it is first turned on (Hey Cortana is fully optional to the users). By training Hey Cortana with your voice, the phone only responds to your commands and not someone else's. Microsoft does not store your voice samples in the cloud, and instead they stay local (for those of you concerned about privacy). You can also wipe the training at any time to start over.
Finally, Hey Cortana is a proper system app, meaning once it is on the phone via the Denim firmware, afterwards Microsoft can dynamically update the app through the Store. This configuration is similar to how Microsoft can update Extras + Info or Display on Lumia devices. It ensures that new features or bug fixes can be applied to phones without the need for an OS or firmware update.
Overall, Hey Cortana is pretty neat and we cannot wait to see more devices pick it up in the coming weeks.