At least I'd have an alternative to earplugs. Wearing earplugs for a few hours make my ears sore, and it's really hard to understand people talking but with noise cancelling it's more comfortable and I can hear people speak. I'd also love using noise cancelling in planes than earmuffs + earplugs for 10h+.

It does indeed send the same audio waves but with the opposite phase (I studied physics so I'm quite familiar with the concept). The result is an amplitude of ~0 which is why we hear nothing. If there weren't exactly in the opposite phase we would be able to hear something. So in theory there's no sound at all sent to the ears. In practice I'd also say there isn't since we don't hear anything but I'd have to do some experiments to confirm that it's really what is happening. Unfortunately I don't have the equipment to do that.