What is it?
Honda's newest offering in the wildly successful VTEC-powered engine series is the i-VTEC. In addition to having a two- or three-profiled cam driving the valvetrain, this incarnation adds continuously variable valve timing. (Well, this is new for a VTEC engine, but not new in the automotive world. Other auto manufacturers have been doing this for several years. Toyota's VVT-i technology does the same thing.) This particular feature is called VTC, or Variable Timing Control.
How it works:
The camshaft is attached to a small hydraulic device containing a helical screw. By varying the oil pressure going into this device, the screw can be made to turn in either direction, taking the camshaft with it.
The end result is that the overlap between the opening and closing of the intake and exhaust valves can be adjusted to suit engine needs.
What's the difference between that and normal VTEC?
Normal VTEC adjusts the lift, or the maximum distance one or both of the intake valves will open, and to a fixed extent, when they open and close. VTC adjusts when they open and close more precisely (it doesn't touch the lift). Normal VTEC provides for better fuel efficiency at low and medium revs, while producing extra power at high revs. The new VTC modification increases medium-band power, which isn't helped by VTEC, thereby answering a common complaint against VTEC.
Where can I get one?
Look for small (2.0-liter and thereabouts) four-banger DOHC i-VTEC engines to show up in the new Acura RSX line, specifically with the Type S package. After that, it will eventually sneak into the engine bays of other Acuras and Hondas.
Props to http://asia.vtec.net/article/ivtec/ for explaining the helical screw thing. Check out the dyno plot!