  In this Letter, we give a detailed analysis to the M3.3 class flare that
occurred on August 17, 2013 (SOL2013-08-17T18:16). It presents a clear picture
of mutual magnetic interaction initially from the photosphere to the corona via
the abrupt rapid shearing motion of a small sunspot before the flare, and then
suddenly from the corona back to the photosphere via the sudden retraction
motion of the same sunspot during the flare impulsive phase. About 10 hours
before the flare, a small sunspot in the active region NOAA 11818 started to
move northeast along a magnetic polarity inversion line (PIL), creating a
shearing motion that changed the quasi-static state of the active region. A
filament right above the PIL was activated following the movement of the
sunspot and then got partially erupted. The eruption eventually led to the M3.3
flare. The sunspot was then suddenly pulled back to the opposite direction upon
the flare onset. During the backward motion, the Lorentz force underwent a
simultaneous impulsive change both in magnitude and direction. Its directional
change is found to be conformable with the retraction motion. The observation
provides direct evidence for the role of the shearing motion of the sunspot in
powering and triggering the flare. It especially confirms that the abrupt
motion of a sunspot during a solar flare is the result of a back reaction
caused by the reconfiguration of the coronal magnetic field.
