Ok, so material burst is getting matter to explode -here probably hydrogene in water drops- with the E=mc2 law.I understand it's use the second time. It's important to leave a scar in your enemy's harbour that prooves they were severely beaten.
But the first use was unneccessary in my opinion. Unless Tatsuya's zapping spell has a size limit, I think it would have been a better option for the first strike.