Yeah so in that video you’re going way too low on the Power lever all the way into the BETA range (you can see it in the cockpit, but not in the external view with those simplified instruments). This means you have reduced power below the “IDLE” position on the lever.
Normally the prop blade angle is controlled by something called the propellor governor which changes the blades angle of attack to match the RPM you set with the RPM lever. However, in the BETA range, the power lever controls the angle of the blades.
BETA mode is only meant to be used on the ground for taxiing or reverse thrust.
Relevant section from the POH:
And this is a helpful diagram showing the blade angles in different modes (these angles aren’t going to be exact for the 208, this is just an example):
So on the ground this isn’t an issue because the plane isn’t moving through the air at 100kts+ but since you’re in the air, that air pushing against you is also going to be forcing the blades to spin and you don’t have the propellor governor changing the angle of the blades to maintain the RPM so it just goes nuts.
Hope that helps!