That is a great video. I understood why it was wrong from the beginning, but it was an interesting walk through. The stuff at the end about impednace matching can be proven, even without the balls and the heavy math. All you need is some strings (low stiffness) of three different diameters. You the the string with the middle diameter and tie it to a fixed point (doesn't matter where, or for all intents and purposes, the length). Flick the loose end of the string, and you will see a a wave go down the length of the string until it hits the end of the string (where it is tied to). If you observe the string when it hits the end (called a return wave), you will see a wave bounce back, and it will bounce back in opposite phase to what you originally flicked it (if you flicked it upwards, the return wave will bounce back downwards, if you flicked it dpwmwards, the return wave will bounce back upwards). If you now tie a string of different diameter to the medium diameter string, and the other end to a fixed point, when you flick the medium diameter string, and it hits the different diameter string, the wave that shows up in the different diameter string will be of opposite phase if the strings are of sufficiently different diameters (stiffness actually).
The first case is called a total mismatch, and it represents the case of which the string energy tries to return up the string back up to your hand. The second case (the two strings of different diameters) is a partial mismatch, and is also of interest, as it is most often seen in the real world. There are a lot of places this is important, aside from being interesting to look at (both the steel balls in the video, and strings experiment), specifically, in RF transmission lines (like the coax cable that comes to your set top box from the cable company or satellite provider). In some cases, you can get distorted colours or ghost images. It can cause signal losses or bad sound quality during long distance phone calls, especially if it is using the transatlantic cables (and it doesn't matter if it is copper cable or fibre optic cable, either, explaining why guys who can splice fiber optic cables properly make the kind of money that they do). Physics enters into a lot of areas, electronics being a big one.
