At high engine speed

The rocker arm oil control solenoid is turned on by the ECM/PCM. Oil pressure from the rocker arm control solenoid enters into 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 secondary rocker arm and the exhaust primary 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 Diagram - High Engine Speed (J37A4 Engine)

Courtesy of AMERICAN HONDA MOTOR CO., INC.