  We present an exceptional data set acquired with the Vacuum Tower Telescope
(Tenerife, Spain) covering the pre-flare, flare, and post-flare stages of an
M3.2 flare. The full Stokes spectropolarimetric observations were recorded with
the Tenerife Infrared Polarimeter in the He I 1083.0 nm spectral region. The
object under study was active region NOAA 11748 on 2013 May 17. During the
flare the chomospheric He I 1083.0 nm intensity goes strongly into emission.
However, the nearby photospheric Si I 1082.7 nm spectral line profile only gets
shallower and stays in absorption. Linear polarization (Stokes Q and U) is
detected in all lines of the He I triplet during the flare. Moreover, the
circular polarization (Stokes V) is dominant during the flare, being the blue
component of the He I triplet much stronger than the red component, and both
are stronger than the Si I Stokes V profile. The Si I inversions reveal
enormous changes of the photospheric magnetic field during the flare. Before
the flare magnetic field concentrations of up to 1500 G are inferred. During
the flare the magnetic field strength globally decreases and in some cases it
is even absent. After the flare the magnetic field recovers its strength and
initial configuration.
