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It depends on what device you are using.
There needs to be a trigger to start the process. It is usually a screen tap. On the Droid Turbo 2 it is a hardware/firmware enhancement. It has one processor dedicated to listening for the launch phrase from the voice of the person who trained it. In my case, screen off. in my pocket, I say "OK Turbo navigate home" and it will launch the app to give me turn by turn instructions on how to get home (while still untouched in my pocket).
I believe other devices are adopting this touchless control. I do not have a list of which ones they are.
... Thom