Instead of going back to when Rainbow Dash performed the sonic rainboom that brought her and her friends together, Twiliight thought back to when her world's Sombra was defeated. When he was, everyone else was crystallized protected against the explosion by the crystal heart. But Sombra wans't just defeated, he was destroyed. That gives her the key to intervening with the war between Equestria and the Crystal Empire. Twilight has to destroy King Sombra in order to stop the crystal soldiers from attacking.