Hit back+taunt and then mash down+taunt until he gets out his rice ball then you unlock this version of Akuma (with a double air fireball and some new supers).

I would never have found this but shoutout to @b_HAUNT who first let me know this was in the game. This is so big that there's an IGN video on it but if anyone knows who first found it, please post it in the comments.

  / deskclips