You mean the VVT solenoid? ("VVL" is Nissan technology BTW)
read here:
http://www.billzilla.org/vvtvtec.htm
^^^ the article is really a comparison of VVT to Vtec, but it explains it pretty well.
VVT can be controlled by means of an RPM window switch or other external controller (VAFC) where other criteria can be used to trigger it.
Typically, to preserve the engine, the ECU's VVT circuitry cannot be engaged when the engine is cold (below 50*C..on the stock dash, minimum temperature for VVT is roughly at the first notch off the cold mark).
On the early model silvertops, VVT was RPM dependent, while on later model silvertops and blacktops it's throttle position dependent. The blacktop supposedly uses VVT on start-up but I've yet to put a light on it and see for myself.
