At high engine speed
WARNING: This page is about a different car, the 2013 Acura MDX, 2012 Acura MDX, 2011 Acura MDX, and 2010 Acura MDX. However, it is still accessible from the selected car via links, so may be relevant.
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 via the rocker shaft, and it moves the VTEC switching piston in the intake secondary rocker arm. This causes the VTEC switching piston to slide into the primary rocker arm, locking the primary and secondary rocker arms together. Both intake rocker arms are lifted by the high lift cam lobe.