There is some false/wrong info and/or mixing up info vs other companies' turbo features so I'm just going to do a detailed explanation; although the vid was good. The following info is about intel's tech as others are different.
Turbo is not really a power saving feature, it's for better efficiency which I understand maybe confused with power saving but this is not so straightforward. Efficiency is using what is available in the best way possible and in this case it's the cpu's tdp. Please note tdp is not power. In nominal speeds it will not fully utilize all its available headroom. Turbo uses this extra headroom to increase performance. However, the cpu can and will go above its tdp during turbo. In some cases turbo does lower power but it can also raise power. As explained in the vid, speedstep is for power saving, it lowers the speed and voltage and lowers the power. But on most desktop cpus, when turbo shifts power around, they also add extra power and thus raise power consumption above stock. They are using the extra tdp headroom and this extra is why turbo doesn't stay on all the time (although it can in many desktops). Similarly most intel cpus have a turbo speed when all cores are active that is above the normal speed. Even if they didn't add this extra, shifting power around would be the same power consumption so again it's not really saving power.
Also mentioned in the vid, their is software that may not fully utilize all cores so turbo can just boost a few cores that are actually being used: more efficiently using the power for better performance and not so much of saving power although finishing the task earlier can get it back to idle earlier and save power. In gaming that's just more fps and more power in the end so it can also depend on the task.
Being activated depends on heat and power and intel has a multi step approach and has a different speeds per active cores. The speed increases will be different on different cpus but for the i5 4670k, turbo bins is 2/3/4/4. So:
With a normal speed of 3.4
4 cores - 3.6
3 cores - 3.7
2 and 1 core - 3.8