Engines are complex bits of machinery.
At their core level, yes, it's easy to understand what is happening. Air and fuel enter the engine where they are ignited and combustion occurs resulting in power that turns shafts.
In a modern engine, there's a whole heck of a lot more going on in there and it's usually related to a computer telling a lot of moving parts how to behave in order to achieve either the best performance, the best fuel economy and emissions, or a combination of the two.
Two of the ways in which this is achieved in a modern machine is through the use of Variable Valve Lift and Variable Valve Timing. If you guessed that these can adjust the timing of the valves and the duration then you're on the right track to getting a good grasp on the engineering behind VVL and VVT systems.
Power, efficiency or both
It's all explained in the video with help from both Jason of Engineering Explained and Charles from Humble Mechanic. The pair have opened up a turbocharged 2.0-liter inline-4 from a 2009 Audi A4. With the heads off, you have a great look at the camshafts and that's where you'll see the magic of both the VVL and VVT systems.
With the Variable Valve Lift system, the camshaft on the exhaust side of the car has two different cam profiles for each valve. There's a short and a tall cam lobe, and it sits on a sleeve that rides along the camshaft. Under shorter loads, the shorter lobe is used but when more engine load is created then the sleeve slides over so that the taller lobe is being used. This increases both the amount of opening creating with the valve and also the duration that the valve stays open. The engine is working harder and this taller profile allows a greater amount of exhaust gas to flow out of the engine, where the turbo will put it to good use.
On the other side of the engine, the Variable Valve Timing system is used to alter the flow of air into the engine. It adjusts the profile of the intake valves, but it does this with a singular camshaft profile. What is altered is the actual timing of that cam in the combustion process. It can move forward and backwards inside of the head of the engine. One direction will advance the timing while the other retards it, and this is done to help with fuel emissions. Letting the air in earlier or later adjusts the combustion process to suit the needs of the vehicle at a given speed and level of engine temperature.
Both of these systems, and more, all work together to make sure your performance cars maximize what they can do, your efficient cars remain just that, and everything else falls somewhere in between based on how the vehicle is being driven.