Board Thread:General Discussion/@comment-4858517-20140913002858/@comment-8986886-20140913022909

One has to activate a Zanpakutou's Shikai, or Bankai, before one can use its special ability, or any techniques connected to it, so when Ichigo used his crossed blade Getsuga Tenshou, Zangetsu must have been in its Shikai, or Bankai, state. Since the two swords had the same design as they had when Ichigo first held them, it's safe to assume that Zangetsu was forced into its Shikai State at that moment, just like what happened to his old sword.