In the last Toei Animation's "Dragon Ball Super" episode, Trunks uses Spirit Sword Bomb and now the fans are wondering what could it be. Especially how does it works and how can it defeat Zamasu. Apparently, it's not a simple weapon and only the purest Saiyan can use it.
"Dragon Ball Super" constantly thrills its fans, the original Goku story is just enhanced in this series. Actually, It is the sequel to the prior Dragon Ball Z manga and anime series where it continues after Majin Boo's attack.
Previously, Future Trunk reappears again in "Dragon Ball Super" episodes where he brings a horrifying news. Supposedly, there is a similar guy with Goku and he is Goku Black or known as well as Zamasu. And, they need to combine forces to relinquish him.
According to Reddit Dragon Ball Super Page, "as things have now reached a point in the Dragonball Universe where powers need to be combined to defeat new foes, the 'Spirit Bomb' seems to be the only move that can defeat them" It appears that "Dragon Ball Super" Trunks really has no more means of fighting.
Otakukart Anime blog explains Future Trunks' Spirit Sword Bomb is the same as Goku's Spirit Bomb technique. Outwardly, it gathers energy from other living things to form a destructive mass weapon.
Although "Dragon Ball Super" Trunks uses differently this attack, reasonably it has more "good will" essence. Additionally, it is the only key to the defeat of Zamasu, he is the epitome of an evil of course only its opposite could destroy him.
Trunk's "Dragon Ball Super" holy Spirit Sword Bomb bombard him in a way Zamasu can't deflect anymore the attack. In essence, only the purest of heart can use the upgraded attack.
In conclusion, "Dragon Ball Super" characters were just made better. And, the fans will see more about the latest upgrades of each character starting with Trunks.