It's basically the EX move of this game, sorry for such a late reply but I think if you hold two of the same attack buttons it's makes a special move into a shadow move.
When somebody is doing a shadow move, there are 5 hits. If you hit the breaker for the move on 3 consecutive hits of the 5 hits of the shadow move, then the announcer will shout "1,2,3! C-c-c-combo breaker!". It's pretty mash-able.