VTEC works by allowing you to have 2 different cam profiles on 1 cam. One that idles right and gives good low rpms performance and another profile with more lift and duration that boosts high rpm performance. Basically the best of both worlds. To my knowledge, the only different between I-VTEC and VTEC is VCT, variable cam timing (I think that's right or VTC?). Basically it's a computer controlled adjustable cam gear on the intake cam shaft. Like a set of cam gears you would buy for a VTEC engine that allow for small changes in cam timing, both do the same thing except VCT isn't static, it can change the settings as the engine is running for better emissions and high rpm performance. I-VTEC engines also spin the opposite direction.