VTEC System

The VTEC system changes the cam profile to correspond to the engine speed. It maximizes torque at low engine speeds and output at high engine speed.
The low lift cam is used at low engine speeds, and the high lift cam is used at high engine speeds.
The K20A2 and K20Z1 engines change both the intake and exhaust camshaft. Only the intake camshaft side has a VTEC mechanism on the K20A3 engine.

Fig 1: Overlap Graph
Courtesy of AMERICAN HONDA MOTOR CO., INC.