It is a barrage of blue or yellow Ki Blasts fired from both hands together.
Future Trunks performs this technique desperately in against Future Android 17 and Future Android 18 after attacking them with his Buster Cannon, but the attack has no effect on the Androids. In the past, Future Trunks used this attack to prevent Semi-Perfect Cell from getting to Android 18, but the attack did little to harm the Bio-Android. However, when Cell absorbs Android 18 and passes by his process of transformation to his Perfect form, Future Trunks fears the worst happen against to the Earth and uses again the attack to try to interrupt such phenomenon, but Cell blocks the attack with an Energy Shield.In Dragon Ball GT, Trunks uses a similar attack during a final investee in conjunction with the Dragon Team against Omega Shenron, but such efforts are useless before the powerful Shadow Dragon.
In Dragon Ball Xenoverse 2, the firing stance of the Charged Ki Blast: Rush Type (or simply Rush Ki Blast) resembles Saiyan Rapid Fire which is fitting as Future Trunks, Kid Trunks, and GT Trunks are all Rush Type, however it can also be used by several other characters as well. The Future Warrior can fire Rush Ki Blasts depending on their currently equipped Super Soul or as their default Charged Ki Blast type if their selected race is Saiyan.
High volume and rate of fire, leaving few openings.Dragon Ball Xenoverse 2 Rush Ki Blast Type description