Singing this Note is IMPOSSIBLE!

Do do do... Just perusing the comments while I have it paused at 1:35.
WHY?! You ask?
Here is what I believe is happening, having only taken High School physics, and learning online some pop-science pseudo-physics.
The frequency of the sound they are making is causing pressure to build up inside the tube. Lower sounds, like in a didgeridoo, can flow more freely out of the tube because they aren't bouncing around so fast, which is why it's easier to make a lower pitch. When you get to the higher pitches, the frequency increases, and pressure builds up to a point where a higher amount of the waves are actually bouncing back into your mouth and into your larynx. When you finally reach the pitch, you've reached a "breakeven" point wherein the air molecules are bouncing at a frequency that is the inverse of the pitch you're trying to produce in larynx, so it comes out muffled.


Since the tube is rather uniformly smooth, the waves are more consistently bouncing back and forth. This is why it doesn't work with your hand, as it's both too short, so you'd have to produce a VERY high pitch, and it's not as smooth a surface, so even if you were able to produce a high enough pitch not enough of the waves would be bouncing back into your larynx. 


A better way to illustrate this would be to imagine you have air molecules in larynx that vibrate in a consistent manner in order to produce the pitch. When the pitch gets high enough, those same air molecules are vibrating in such a way that actually "cancels" out the pitch you are making. Active noise-cancelling headphones do the same thing. They produce an inverse sound wave in order to cancel out the incoming sound waves.


It's either that, or the wavelength of the pitch nearly perfectly matches the size of the tube, causing it to "miss" the tube entirely when it comes out. Since you're used to the pressure beforehand, the instant release of pressure causes you to make that sound because you're pushing more than you needed to at that moment.