  We report a new "dip" effect in the Hall resistance, R_{xy}, of a Si
metal-oxide-semiconductor field-effect transistor in the quantum Hall effect
regime. With increasing magnetic field, the Hall resistance moves from the
plateau at Landau filling factor \nu=6 directly to the plateau at \nu=4,
skipping the plateau at \nu=5. However, when the filling factor approaches
\nu=5, the Hall resistance sharply "dives" to the value 1/5(h/e^2)
characteristic of the \nu=5 plateau, and then returns to 1/4(h/e^2). This is
interpreted as a manifestation of the oscillating exchange enhancement of the
valley splitting when the Fermi level is in the middle between two adjacent
valley-split Landau bands with the asymmetric position of the extended states.
