How to make Google Now run continually in the background on Android

I have a app idea in mind, but I haven't been able to figure out one part yet. In Google Now on Android, it gives you the following options to search:

1. Type to search
2. Click microphone to say your search
3. Or just Say "Google" to make the voice search begin.

I'd like to know how they made #3 work. I need to have a keyword, that when said, initiates the voice search.

I imagine they have some type of background running service constantly listening for voice actions. Does anyone know how this is achieved?

Here is a image of the Google Now app:

You can write a service and then use createSpeechRecognizer in that service. For a complete service code for this you can find the answer at Android Speech Recognition as a service on Android 4.1 & 4.2