The videos seem to omit certain info which makes it harder for me to understand.

As far as I know, if you take a pure sine and make it short in duration then you're actually adding frequencies to it. Called amplitude modulation perhaps? At least this is the same thing.

The only way to have a frequency 100% that exact frequency is to have it at the same amplitude for an infinite amount of time. So for a sine to appear from silence up untill a certain volume and then fade out to silence again makes the sine have an extra frequency part and makes it no longer a pure sine at one exact frequency. The faster you fade in and out the sine the higher the added frequency (or frequencies, depending on the "waveform" of the amplitude change) is.

In the video this is talked about only as if this is the uncertainty principle, but what I describe above is not an uncertainty thing but a real physical effect. Hence my personal confusion what part is real and what part is uncertainty making the video less than ideal for me to understand the principles better. Nice videos otherwise though!

edit: Nevermind, I understand how to integrate what I write with the info given in the videos now. The fourier transform does seem to show the effect I talk about? The spike in the videos at 0sec when they start the waveform not at 0 but at max is actually real as it displays the frequency effect of suddenly starting the waveform (an impulse like beginning, like you hear when you start a waveform not at 0 crossing but at max).

edit 2: Well very confusing to call it uncertainty as it's not uncertainty it's real. Except for the quantum mechanical example though where it is still real in describing the mathematical side of quantum mechanics which describes "potential", not the real measureable outcome which is our reality.