This will prove useful if you are listening in a noisy environments where you might actually need to pay attention to the occasional noise. If you are shopping hearing the sound of gunfire might be rather useful as would be screams of people in distress.

In addition, the headphones can also listen for phrases to turn noise canceling back on again, so the user can resume their listening experience hands-free.

Of course it is just a patent and there is no indication that Amazon has got it working, or intends to release a product around it.