How did Future Trunks learn the Spirit Bomb Technique? In episode 66 of Dragon Ball Super, Trunks gathers energy from the life forms and forms a medium-sized Spirit Bomb while fighting Zamazu. I don't recall Future Trunks seen Goku use Spirit Bomb in the anime, I'm not sure though. How did he managed to do it if he didn't taught by anyone or possibly didn't even seen or knew it in the first place? Moreover, he didn't even concerntrate to gather the energy like Goku did everytime.
The answer is quite simple: This is not the spirit bomb technique. 

Trunks' new form that is similar to the SSB is solely responsible for pulling in all of that energy and enabling that new sword technique he used against Zamasu.

Considering that we don't have a definition for this form we also don't know it's limitations. 

However, I speculate that the new form is a special form of SSB attained when a half human half saiyan can clad themselves in god ki. Doing so enabled him to pull on the ki of other humans to unite into his blade. Which worked because humans are according to Zamasu "made in the image of gods". 

That's only a theory but fact of the matter is that what we saw was not Trunks using the spirit bomb.