  The sub-cycle dynamics of electrons driven by strong laser fields is central
to the emerging field of attosecond science. We demonstrate how the dynamics
can be probed through high-order harmonic generation, where different
trajectories leading to the same harmonic order are initiated at different
times, thereby probing different field strengths. We find large differences
between the trajectories with respect to both their sensitivity to driving
field ellipticity and resonant enhancement. To accurately describe the
ellipticity dependence of the long trajectory harmonics we must include a
sub-cycle change of the initial velocity distribution of the electron and its
excursion time. The resonant enhancement is observed only for the long
trajectory contribution of a particular harmonic when a window resonance in
argon, which is off-resonant in the field-free case, is shifted into resonance
due to a large dynamic Stark shift.
