It's hard to describe but with my Sony's, on a plane with it turned off you hear the regular engine noise, with them on the sound changes to kind of like a low volume, low frequency rumble type thing. The effect (or whatever) definitely works on a plane, and very well. My big problem is remembering to take them, it's the only time I use them.

I don't understand what your confusion is.

Let me explain it a different way.
Cold water out of your fauces is 50°
You want 100°
You add equal volume of 150° water and end result is 100°
You're not exposed to 150° temperatures, just the 100° you asked for.

Point taken... I can still hear!

He might be thinking of the analog of this, which would be that there's a flaw in the piping and there's a place where you're exposed to 150° water.

Nope. The volume of air into which the opposing sound comes is so small that there is, in effect, no place where the opposing sound is "by itself," so you don't hear it. You only hear the result of it being mixed with the original sound.

To have such an effect, you'd have to be doing this in an enclosure where the wavelength of a frequency is an appreciable fraction of the room size. Let's say the top frequency here is 100 Hz; sound travels at about 1000 feet per second, so the wavelength of 100 Hz is about ten feet. Wavelength of 1 kHz is about a foot.

The space inside your headphone/pinna/earhole complex might be as long as an inch, which is the wavelength of about 12 kHz. So.... you see how the sounds won't ever be separate in that enclosed space?