  Using one-dimensional semiconductor Bloch equations, we investigate the
multiband dynamics of electrons in a cutoff extension scheme employing an
infrared pulse with additional UV injection. An extended three-step model is
firstly validated to play a dominant role in emitting harmonics in the second
plateau. Surprisingly, further analysis employing the acceleration theorem
shows that though harmonics in both the primary and secondary present positive
and negative chirps, the positive (negative) chirp in the first region is
related to the so-called short (long) trajectory, while that in the second
region is emitted through `general' trajectory, where electrons tunnelling
earlier and recombining earlier contribute significantly. The novel
characteristics deepen the understanding of high harmonic generation in solids
and may have great significance in attosecond science and reconstruction of
band dispersion beyond the band edge.
