WARNING: The following contains spoilers for My Hero Academia: Vigilantes Volume 8 by Hideyuki Furuhashi, Betten Court, Kohei Horikoshi, Caleb Cook and John Hunt, available now in English from Viz Media.
My Hero Academia's Shota Aizawa is well known for his unique power -- the ability to "erase" the Quirks of others. But Aizawa's Quirk works best when his opponents don't expect it. Here are five secrets about Erasure that Aizawa would rather keep to himself.
Aizawa Only Needs One Eye Open to Use Erasure
It's well known that Aizawa needs to maintain eye contact with the subject of his Quirk, meaning blinking is his worst enemy. This automatic human motor function can spell out huge issues on the battlefield, as Yaoyorozu and Todoroki proved during the practical exam. They constantly used their Quirks, so that they knew when Aizawa had spotted them, and so they could take action the millisecond their Quirk activated before he opened his eyes again.
But that doesn't have to be the case. While it would take focus and discipline bordering on superhuman, the fact is that Aizawa only needs one eye to activate his Quirk, so if he blinked eye individually, he could keep his Quirk effective indefinitely. Obviously, there are some other factors that can cause an upset, such as the villain ducking out of view, creating a smokescreen or even his own dry eye forcing him to "blink" more rapidly. However, so long as he can see them, they're goin' down.
Aizawa's Field of Vision Is Also Erasure's Field of Effectiveness
It was revealed during the USJ attacks that Aizawa's Quirk works on multiple opponents at once. While he needs to keep his eye on the targets, he can put a stop to any team-plays they may have been planning. Further helping this is the fact that he only needs to look at some part of their body for his Quirk to be effective, meaning he can nullify the Quirks of entire crowds at once, despite not necessarily being able to make out individuals. Distance is generally good for Aizawa, allowing him to see more people and thus cancel more Quirks, but is also a double-edged sword, since with such a range he could accidentally erase allies' Quirks as well. In the same way, the further away from the target he is, the easier it is for them to run and hide out of sight.
Just because he's able to negate the Quirks of every opponent at once doesn't mean he can't be defeated. A person's natural non-Quirk enhanced strength, agility and mind are completely unaffected by Erasure, so a depowered but strong opponent will still put up a fight. This is proven by the USJ Nomu, who, despite having its Quirk erased, was still able to snap Aizawa's arm.
Aizawa's Quirk Is Even Effective Behind Glass
Aizawa's Quirk is even effective behind glass or clear plastic. This was proven in the spin-off prequel manga My Hero Academia: Vigilantes Volume 8, Chapter 61, where he borrows a pair of goggles from then-classmate Shirakumo. He does this to protect his eyes from the smoke emitted by the villain and proves that as long as he can look directly at you, Erasure will be effective. He's since changed up the design of his goggles to focus more on misleading his opponents by obscuring their vision of his eyes, making them unsure of whose Quirk he's canceling.
This means Eraser could easily take up a position similar to a sniper, taking out Quirks from a distance through binoculars and windows while other heroes go in, staying away from the windows so as to continue using their Quirks, and take the villain down before they even know what's happening.
The Erasure Quirk Has Its Limits
Unfortunately, Erasure is not without its drawbacks, and Aizawa's chronic dry-eye is the evidence of that. Despite only needing one eye on his target at a time, the human instinct is to use two, something that has come back to bite Aizawa in the butt. Constant overuse of his Quirk with both eyes has made the most valuable support item Aizawa has not his high-tech capture cloth -- but the everyday eyedrop. Couple this with the injuries he sustained during the USJ arc, and it's easy to see why the amount of time Aizawa can use his Quirk has shortened significantly since the beginning of the series, and why, more often than not, he needs to rest between uses.
His Quirk-erasing abilities are not absolute. Heteromorphic-types, aka those with "modified" bodies, are more or less immune to his ability, only finding themselves unable to use certain body parts that wouldn't normally be found on the human body, such as a tail. This is because Erasure doesn't actually erase Quirks as Aizawa revealed in My Hero Academia Volume 15, Chapter 135. It simply halts the "Quirk factors" that allow it to work. Erasure is only temporary and does no damage to the Quirk, meaning that even if Aizawa was able to get close to a major villain and use Erasure, he can't make it permanent.
Erasure Has An Unexpected Side-Effect (That is Easily Negated)
Whenever Aizawa activates his Quirk, his eyes glow red, and his hair begins to fly -- and while he's already found a way around the glowing eyes with his goggles, his hair is the tell that villains use most against him, being noticed by Shigaraki and used against him during the USJ attack. Strangely enough, this odd side-effect is also the easiest to manage!
A simple haircut will solve the problem, as a buzz-cut would make it impossible for villains to tell whether or not his hair was standing on end, thus further concealing when he uses his Quirk. While he claims to live rationally, we're surprised he hasn't gone for a quick trim sooner.