But... For those that can ... what is the effect one can obtain ?
Play this back at 2X speed, in mono, and without the artificially sounding echo-y effect(I can collapse any audio into mono with a button press from my RME, but removing the echo-y aspect I wouldn't know how to do without Equalizer APO for example, but it's nothing too serious, as the sound is still pretty accurate to what I hear when I flex that muscle). That's all that occurs, with more effort exerted, it just becomes louder.
If you need an actual demonstration personally, just stick your fingers in your ear and pay attention to the rumbling sound from what I presume is some sort of occluding effect. (This can also be achieved with IEM's when you get a seal)
I feel this also happen out of my control when there is an extremely loud high frequency sound being played (like deafeningly loud for example), it seems to be what I assume is a defense mechanism against damage to such sounds.