  The bi-circular scheme for high harmonic generation, which combines two
counter-rotating circular fields with frequency ratio 2:1, has recently
permitted to generate high harmonics with essentially circular polarization,
opening the way for ultrafast chiral studies. This scheme produces harmonic
lines at 3N + 1 and 3N + 2 multiples of the fundamental driving frequency,
while the 3N lines are forbidden owing to the three-fold symmetry of the field.
It is generally established that the routinely observed signals at these
forbidden harmonic lines come from a slight ellipticity in the driving fields,
which breaks the three-fold symmetry. We find that this is neither the only nor
it is the dominant mechanism responsible. The forbidden lines can be observed
even for perfectly circular, long driving pulses. We show that they encode rich
information on the sub-cycle electronic dynamics that occur during the
generation process. By varying the time delay and relative intensity between
the two drivers, we demonstrate that when the second harmonic either precedes
or is more intense than the fundamental field, the dynamical symmetry of the
system is broken by electrons trapped in Rydberg orbits (i.e., Freeman
resonances), and that the forbidden harmonic lines are a witness of this.
