  The low mass X-ray binary GX 339-4 went into a new outburst in 2021. At the
end of the hard to soft transition of this outburst, Insight-HXMT found that
the source rapidly alternated between low flux and high flux states on a
timescale of hours. Two high flux states lasted only for a period comparable to
the orbital period of the observatory. Time-resolved spectral analysis shows
that the sudden changes of flux are confined in the hard X-ray band (>4 keV).
The variable non-thermal emission, including the power-law continuum from the
corona and the reflected emission from the inner accretion disk, is responsible
for the observed variability. The strength of disk thermal emission and the
inner radius of the accretion disk are consistent between the two flux states.
Assuming the lamppost geometry, our best-fit disk reflection models suggest a
very low corona height (within 3 $R_{\rm g}$) and there is no evidence of
significant variation in the corona geometry either. The observed rapidly
alternating flux states suggest that the intrinsic power of the corona must
change during the state transition. We discuss possible mechanisms for the
observed sudden changes in the coronal power of GX 339-4.
