i didn't feel it "kick in" like a turbo lag or nitrous, but i can hear and feel the valves fully opening up and the extra air passing through the intake manifold on my friend's b18, c32, and my f20. The engine tune also changes when vtec "kicks in". I guess the torque curve is a lot smoother on my f20 compare to other engines so i don't feel a jolt when vtec kicks in.
vtec is an engine management that controls the valve timings at low and high rpms. Compare to toyota's vvt-i, vtec is mostly tuned for and vvt-i is mostly tuned for fuel economy. However they're both designed to save fuel at low rpms and maximize power potentials at high rpms.
vvtl-i however is the same as i-vtec, since vvt-i is fairly old and inferior compare to i-vtec.
and to all fellow honda drivers: when you race, never drop out of the vtec range.
once i was on the freeway and i didn't downshift enough gears....i got raped by an eclipse !@!$@#!
and once my friend's rx7 raced me and lost....all he could say was "f*cken vtec"