The intake manifold has a dual intake runner setup, versus a single intake runner setup like most other engines have. The "secondaries" are referred to as the second set of intake runners that are opened up by the IMRC (Intake Manifold Runner Control) at around 3200RPM at WOT (Wide Open Throttle) or 3900RPM at partial throttle. The intake runners are what provide air to the cylinder combustion chamber. The "secondaries" are a shorter and larger diameter set of intake runners designed to provide a faster and larger charge of air into the combustion chamber at higher RPM's (versus the intake runners that are always open at low RPM - which are smaller diameter and longer for smoother airflow). In short, it's why you feel that whoosh of power after 4k RPM in the Duratec powered cars...
Hope this helps...
