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 into the intake secondary rocker arm and the exhaust primary rocker arm via the rocker shaft, and it moves the VTEC switching piston in the intake primary rocker arm and the exhaust secondary rocker arm.

This causes the VTEC switching piston to side into the secondary rocker arm, locking the primary and secondary rocker arms together. Both rocker arms are lifted by high lift cam lobe.

Fig 1: VTEC System Diagram (High Engine Speed)

Courtesy of AMERICAN HONDA MOTOR CO., INC.