I didn't watch the Comp Cams video yet, but I think the kid misses a big point about the dynamics.
It's all about torque production. Torque production is all about how well the cylinder fills with fuel/air mixture. If that mixture weren't elastic and didn't have momentum, you could simply have 180 degrees of duration on intake and exhaust. You could open the intake valve at 0 (TDC) and the mixture would rush in and fully fill the cylinder at 180 (BDC). You could then close the valve, compress, and fire. Back at 180 (BDC) of the power stroke, you could open the exhaust valve, closing it at the same time the intake valve opened at 0 (TDC).
However, air and fuel have mass, and they are elastic. So when you open the valve, it takes time for the mixture to accelerate, and there's a delay after BDC when the piston is coming up, but air is still rushing in because of elasticity and momentum. That's why you extend duration after 180 degrees.
Back up the column of air, in the intake, that air also has mass and momentum. It's all going down the intake tract and doesn't want to stop. So you can open the intake valve before the piston has reached TDC, and some mixture will still try to flow in because of momentum and compressibility.
As you increase engine speed, these things happen faster and faster, so you have more overlap and more duration to allow for the mixture to fill the cylinder at these higher speeds. You don't typically make more torque, you just make it at higher revs. Since HP is torque x rpm, you make more power. The down side is that you need the mixture moving at high speed, with lots of momentum, to make it work. If you don't, like at idle or low rpm, you get poor cylinder filling and poor torque.
The more you open the valve, and the faster you open the valve, the better your cylinder will fill. The better the cylinder fills - at any engine speed - the better the torque you make. You are limited in this by a number of things, such as lifters, valvetrain mass, spring rates, and duration (the longer you have to do the action, the more lift you can accommodate).
So for a given lift, more duration won't change the torque output much (ignoring mismatched parts up and downstream), it will just increase the RPM where it is produced. It will make more power (HP=RPMXTorque/5252), but less of that power will be lower in the rev range.
