  The Helioseismic and Magnetic Imager provides 45 s cadence intensity images
and 720 s cadence vector magnetograms. These unprecedented high-cadence and
high-resolution data give us a unique opportunity to study the change of
photospheric flows and sunspot rotations associated with flares. By using the
differential affine velocity estimator method and the Fourier local correlation
tracking method separately, we calculate velocity and vorticity of photospheric
flows in the flaring NOAA AR 11158, and investigate their temporal evolution
around the X2.2 flare on 2011 February 15. It is found that the shear flow
around the flaring magnetic polarity inversion line exhibits a sudden decrease,
and both of the two main sunspots undergo a sudden change in rotational motion
during the impulsive phase of the flare. These results are discussed in the
context of the Lorentz-force change that was proposed by Hudson et al. (2008)
and Fisher et al. (2012). This mechanism can explain the connections between
the rapid and irreversible photospheric vector magnetic field change and the
observed short-term motions associated with the flare. In particular, the
torque provided by the horizontal Lorentz force change agrees with what is
required for the measured angular acceleration.
