AVCS
This article previously incorrectly gave details for Subaru's i-AVLS system, which actually is already referenced at AVLS. (See links to Subaru's website, below)
The implementation of AVCS is quite different to AVLS. The latter just changes which cam is operating one of the two valves.
In AVCS, the engine computer (ECU) can command a solenoid which advances or retards the camshaft rotation by up to 35 degrees.
By retarding the cams at idle or very low engine loads, you achieve a smoother, easier idle. From idle through medium engine loads, AVCS advances the intake valves to begin opening during the last part of the exhaust stroke, when the exhaust valves are still slightly open. Some of the pressure created during the exhaust stroke flows into the intake manifold, having the effect of exhaust gas recirculation (EGR). The intake valves also close earlier during the intake stroke. This helps with engine efficiency and fuel economy.
At very high engine loads, AVCS advances the intake valves further to open even sooner during the exhaust stroke. This produces a scavenging effect – that is, intake airflow helps clear the cylinder of exhaust gas. It also closes the intake valves sooner on the compression stroke. This results in improved volumetric efficiency and helps to generate higher power output.
Read more about this topic: Active Valve Control System