At high engine speed

At high engine speed, the rocker arm oil control solenoid is turned on by the PCM. Oil pressure from the rocker arm control solenoid enters the intake secondary rocker arm and the exhaust primary rocker arm via the rocker shaft, and it moves the VTEC switching piston into the intake primary rocker arm and the exhaust secondary rocker arm. This causes the VTEC switching pistons to lock both rocker arms (primary and secondary) together. Both rocker arms are then lifted by the high lift cam lobe.

Fig 1: VTEC System Operation At High Engine Speed

Courtesy of AMERICAN HONDA MOTOR CO., INC.