The whole thing can get quite complex so I'll try to keep it as simple as possible. This is my favorite site for Valve Timing information:
http://www.ukcar.com/sframe.htm?/features/tech/Engine/vv/vvt_1.htm
There are 3 commonly adjustable variables to Valve timing:
Valve Lift
Open Duration (how long it opens)
Phasing (when the valve opens and closes)
Your correct that the valve must open in conjunction with the cylinders, but when and how long the valve is open is a bit of a science. At high engine rpm the fuel mixture does not flow the same into the cylinders as at low rpm. To allow the ideal combustion to happen, opening the valve slightly sooner helps the fuel get into position sooner. That is what the Z's CVTC does. It’s continuous because at any given rpm the valve may open a little soon, a lot soon, or anywhere in between, depending on what the engineer who designed it deems ideal for that rpm. This controls how much fuel gets into the cylinder and prepares it for combustion better over the rpm range than fixed valve timing, therefore creating more power.
Valve lift and duration also have large effects on this, which I won't go into because the Z's CVTC does not have control over these two factors.
You can find many posts on this with a search for "Valve Timing" or "CVTC" also
