is a side-stepping motion
. Loosely translated it means "to make an unexpected change", and the move can often be seen right at tachiai
(the initial rush at the beginning of a sumo bout): one of the wrestlers may "do a henka" and slap his opponent to the ground with eg. a hatakikomi
(one of 82 kimarite
or sumo winning techniques
A sumotori (sumo wrestler) can use the move to evade his attacker at any given time during a bout, but the henka is probably most closely associated with a spectacular win.
My sources are http: www.csse.monash.edu.au/cgi-bin/cgiwrap/jwb/j-e/dict - and lots and lots of sumo on TV.