Motorola changed the way users interact with their devices when they introduced their "always listening" feature on the Moto X. When I first saw it demoed, I couldn't wait to get the same functionality on one of my handsets, and now that has finally come to fruition.
OnePlus teased the same feature for the One when it was first announced, but it was noticeably missing at launch. Now, OnePlus user jojo_whit3 figured out a way to port the feature over from early builds of CyanogenMod 11S, and after some tinkering, he was successfully able to get the "Hey, Snapdragon" hot word recognition to work.
This feature is ported from an earlier build of CyanogenMod 11S and, because of that, it is incompatible with some of the features included in custom kernels and ROMs. That means that if you are using an AK or Franco kernel on your One, you'll run into force-close issues when installing this.
If you don't mind losing the features that come with your custom kernel, you can flash the stock kernel in TWRP, just as long as you are using a CyanogenMod 11S-based ROM.
Additionally, you'll need to disable Google Now's hot word detection from its settings menu to prevent any conflicts.
Once you finish downloading the ZIP file below, you'll need to boot into TWRP. The fastest way to do that would be to enable Advanced Reboot from your Developer Options, then selecting Recovery from your Power menu.
Tap Installin TWRP, then then select the ZIP you downloaded earlier, which should be in your Download folder unless you moved it. Once the installation has completed, tap Reboot System.
When your phone has booted back up, you'll need to train your device to recognize your voice, which can be done through Settings -> Voice Wakeup.
The training is set up just like Google Now, so after saying "Hey, Snapdragon" three times, you'll be ready to go. If you do not want Google Now to launch every time you say the keywords, you can change it to a different app by selecting a new Activity to launch.
Know that in order to have this phrase automatically open Google Now, or any other program, from the lock screen, you'll need to disable any form of a keyguard security. If you do decide to keep your device protected with a passcode or pattern, you will need to manually unlock your device before your desired app is opened.
Hopefully when OnePlus realizes that this feature has an unofficial version floating around, they'll finally release a full-featured one in their next OTA.