How did Future Trunks become so powerful?
In Dragon Ball Super Episode 57, Trunks fought alongside Goku against Super Saiyan Rose Goku Black and The Immortal God Zamasu.
We see Fuutre Trunks did quite well in the battle even though his power is limited as compared to Goku and Vegeta. Trunks blocked the Ki Sword attack of Black using his Sword, and landed multiple legit hits on Black throughout the battle. Also he was able to land powerful moves on Zamasu however it went in vain as Zamasu is immortal and was not affected by those attacks.
Now, this made many fans confused as to how Trunks became so strong? It appeared as though he was fighting as good as Super Saiyan Blue. Black totally defeated Vegeta and Goku also failed to hurt him. So among Gods how Future Trunks remained helpful?!
The thing is Trunks did get stronger than before by training with Vegeta and learning moves like The Final Flash. But he is nowhere near the level of SSB or SSR. He was able to attack Black the same way SSJ 2 Vegeta was able to attack Beerus. Trunks unleashed his rage and used most of his energy in 1 or 2 attacks. While Black was perhaps holding back and didn’t expect Trunks to attack like that. So the surprise element was at work.
So, just because SSJ2 Vegeta landed some good blows on Beerus or SSJ2 Trunks fought well with Black. Doesn’t mean they were able to keep up or is as strong as them. It comes down to how much power was being used behind their moves. Like, ask yourself this question. If Trunks was fighting Black one on one would he win?
Absolutely not! Keep that apart, if Vegeta didn’t interfere in time both of them would be dead. And we never saw Black’s full power. He never reached a point where his limits were questioned. He didn’t get angry or power up to his max. In a normal fight scenario, if Trunks used one or two moves like that his energy would run out fast and he would get killed. Thus, we can’t even say that he kept up well with Black.