The following contains major spoilers for My Hero Academia Chapter 368, "Rev Up, One For All" by Horikoshi Kohei, Caleb Cook and John Hunt, available now in English from Viz Media.
Speed is one of the most important assets for Pro-Heroes in My Hero Academia, and although One For All's base ability gave him a pretty decent speed boost, Deku has always prioritized getting even faster with his growing stock of Quirks.
In Chapter 368, Deku finally revealed the highly anticipated second user's Quirk. It's an ability that drastically increases the user's speed upon activation, giving Deku access to speeds that even All Might couldn't achieve at the height of his power.
What Is One For All's Second User's Quirk?
Deku's seventh and final Quirk is called Transmission, granting him the ability to modify the speed of what he touches. So far, Deku has only used this Quirk on himself to increase his speed, but based on its description, it's entirely possible that he may be able to transfer its effects to other objects, potentially other heroes. Should this hold true, Transmission would fit perfectly into Deku's preferred combat style. Being a Quirk activated by touch, his rapid fire punches could either slow down or speed up opponents as Deku wished.
The limited use of Transmission shown so far has already cemented it as the most impressive of all the auxiliary Quirks hidden inside One For All. It appears to be able to increase his speed in three stages, each significantly faster than the last. By the third stage, Deku's fighting speed is strongly reminiscent of the flurry of punches All Might punished Nomu with at USJ. He was already moving faster than the eye could follow, and Mirio remarked that the sounds that followed his punches were delayed, implying that he'd already blown past supersonic speeds.
How Is Deku Now Faster Than All Might?
All For One recognized the second user's Quirk, but was taken aback by how much faster it had become since he last faced it decades ago. Even with speed and reflexes that rivaled All Might, All For One was helpless in the face of the young hero's relentless attack, but Deku was only getting warmed up.
He managed yet another increase in his speed by pushing Transmission into Overdrive and at this point, he was entirely invisible to all participating in the battle, including All For One.
Transmission's sudden surge in power may have surprised All For One, but it wasn't just a random power-up given to the Quirk. Like the rest of the One For All user's original Quirks, Transmission had grown stronger in its years of dormancy within the power stocking Quirk. What had previously been a slight boost in mobility that All For One first encountered had morphed into the supersonic ability that was harassing the Demon Lord at the height of his power.
Even though Deku is already much faster than the average Pro Hero, Transmission is a very welcome addition to his arsenal of Quirks. Like both Hawks and Endeavor stressed, speed is essential for Pro-Heroes as it could spell the difference between successfully saving a life and arriving at the scene after the unfortunate fact. Deku's already proven its worth against All For One. Just a couple My Hero Academia chapters ago, the villain was easily brushing off the combined efforts of several Pro-Heroes. Right now, the tables are officially turned.