Yasuhira Koga is a former Clan Yarikawa retainer and swordsman who sides with the Mongols to get revenge on Lord Shimura. He is the villain of the Mythic Tale The Heavenly Strike, where he hunts the same technique you do.
Jin retraces Yasuhira's steps, compares the Mongol version of the legend with Yamato's and follows the trail of white leaves to Shigenori's Rest. Yasuhira forces a peasant's grandfather there to teach him the Heavenly Strike, then kills him.
Yasuhira tests the Heavenly Strike on Jin. Watch how he uses it, because Jin learns it during the fight. Jin turns the move on him and lightning strikes him dead.