Closer view & cutaway of the VVT controller
So, when the computer signals for the VVT to operate, the OCV opens and thus causes the VVT pulley to advance the inlet cam timing by 30°, reference the crankshaft. (15° on the pulley itself)
The rpm at which this happens is worked out by running the engine on a dynamometer with the inlet cam in both the fully advanced and fully retarded positions. Since the two different cam timing's will make different power throughout the rev range, (advanced inlet give more top end power at the expense of low end power, and vice-versa) there is a point where the power will be identical for both cam settings, and this is where the VVT is programmed to operate. Because the power output is the same with the VVT in either position, you can't feel anything when it happens. You can, however, hear a change in engine note, just before there's a big increase in power!
More detail on the the VVT logic - The VVT comes in three types for the 20 valve.
To the best of my knowledge, silvertop 20v's pre May 1993 have the VVT actuate at about 4400rpm. Post May 1993 they seem to work on throttle position and ignore revs.