After dozens of chapters of build-up, My Hero Academia is finally ready to reveal the Quirk that once belonged to the Second User of One for All, and it turns out that this wildly dangerous Quirk was left out for so long for a reason.
Deku began to manifest One for All's stored Quirks starting with Blackwhip back in chapter 210, and proceeded to unlock all but two--as it turned out, the vestiges of these Quirks' original owners didn't trust Deku fully yet. By the time of Dark Deku's vigilante era, he won them over, and My Hero Academia revealed each quirk of One For All, with the Third User's Fa Jin the last one shown to readers. This left one remaining Quirk, the Second User's, a mystery, and its absence went unaddressed until chapter 349, where Deku attempted to use it but was told to hold it back as a "last resort," without further elaboration.
In chapter 368, that "last resort" situation has finally arrived as Deku squared off with the Shigaraki/All for One hybrid being. He naturally yells the name of his attack, calling it "The Second's Transmission" (thus eliminating any ambiguity) and begins to punch into Shigaraki/All for One, increasing the power from Second Gear to Third Gear to Top Gear to its final Overdrive, delivering a Detroit Smash which (according to the Second User's vestige) is approximately 120% of One for All's full power. All for One recognizes the devastating technique as having belonged to an adversary from his distant past, finally realizing what's going on. The vestige of Yoichi Shigaraki, All for One's younger brother and the First User of One for All, appears, declaring their intent to finally bring this century-spanning family struggle to an end.
What Makes The Second User's Quirk So Dangerous?
The Second User's Quirk, Transmission, seems to work as a power multiplier by altering the speed at which objects it touches travel, causing massive and instantaneous acceleration. By delivering what appeared to be a full power strike, Deku was able to exceed his 100% level, which was already a tremendously powerful attack. Prior to using it in 349, the Second User warned that as more power was stockpiled within One for All, his power had mutated and grown, making it highly dangerous to the user. Now that it's been used, it's easy to see why: Deku was regularly shattering his bones long before he even got close to using 100% of One for All's power, so 120% is certainly going to put his health at risk again. The Second User's five-minute warning makes it seem likely that employing Transmission may put a hard cap on Midoriya's fighting time simply due to recoil damage. Combining it with My Hero Academia's other attacks that boost output, like Fa Jin, could be even more devastating, both to Deku and his opponent.
With the last Quirk inside One for All finally revealed, it appears that all of Deku's cards are now on the table. That leaves just one big question for My Hero Academia fans: will these powers be enough to finally defeat All for One and Shigaraki?