  The major solar event on 2006 December 13 is characterized by the
approximately simultaneous occurrence of a heap of hot ejecta, a great
two-ribbon flare and an extended Earth-directed coronal mass ejection. We
examine the magnetic field and sunspot evolution in active region NOAA AR
10930, the source region of the event, while it transited the solar disk centre
from Dec. 10 to Dec. 13. We find that the obvious changes in the active region
associated with the event are the development of magnetic shear, the appearance
of ephemeral regions and fast rotation of a smaller sunspot. Around the area of
the magnetic neutral line of the active region, interaction between the fast
rotating sunspot and the ephemeral regions triggers continual brightening and
finally the major flare. It is indicative that only after the sunspot rotates
up to 200$^{\circ}$ does the major event take place. The sunspot rotates at
least 240$^{\circ}$ about its centre, the largest sunspot rotation angle which
has been reported.
