How do you think Tella used Untell to stop that bullet? They probably explain it later, but I want to know how you think it works for what we have seen.
I'm pretty sure that I addressed this in my chapter review, but since you asked, I'll go into a bit more explicit detail here
We've known since the Winter fight that Tella's ability allowed him to block attacks, as he used his speakers to intercept the Union's attacks, and I guessed at the time that because his name was "Tella" that his ability had something to do with communication (further implied by him being a communication's expert, as mentioned by Billy at the time)
My interpretation at the time was that Tella was negating the "message" that was being sent to him in the form of an attack; "you're our enemy" or "we want to hurt you" being communicated through combat. It's a pretty common trope for fighters to "talk with their fists," so this seemed like a pretty literal interpretation of that idea. From this, I thought that his ability negated any incoming communications
However, that didn't explain why Tella only communicated with speakers that were clearly attached to his head; he could clearly hear what other people said to him, he doesn't use his mouth, it's pretty obvious that he's a self-targeter, and I couldn't figure out how to get around that inconsistency
Now we know that Untell specifically negates forms of communication that use Tella's body. At a glance, this means that his mouth, hands, and any other part of his body that he might use to send a message (such as blinking in Morse code) are rendered incapable of doing so, but that wording implies that Untell only negates outgoing messages
Because Tella blocked Fuuko's bullet and didn't freeze up, that means that the communication being negated was the incoming message from Fuuko, not the outgoing message of "I want to protect Billy"
In fact, Tella then proceeds to take a threatening stance, complete with bared teeth, to communicate with Fuuko "I won't let you touch Billy." That's an outgoing message communicated through body language! And yet it's not negated for some reason
And again, Tella can still hear what is being said; he can still be communicated with, he just can't speak/sign, and he can't receive an attack
My best guess is that he can only negate voluntary communications; he can't help but hear what other people say any better than he can control Billy's ability to hear the beating of his heart. He can control what he says, but if he's trying to defend Billy, he's probably not thinking about what his expression is saying, he's probably only thinking about saving Billy, so his threatening stance is a completely involuntary response
An incoming attack is a deliberate choice, though, a purposeful message with the intent of telling its recipient to die. If someone just carelessly threw a grenade in Tella's general direction, he probably wouldn't be able to block it because there's no intention of communicating a message at all, it's just a random incident with no meaning
Whether this is accurate at all, I'm not sure, but it's my best guess for now. Hopefully Tozuka will give us more helpful info soon!
Basically, it's a case where English word doesn't entirely fit the Japanese meanings. The closest explanation English speakers can get is that "Tell" means "Transmission" or "Connection", which Tella's ability negates.
This a meaning for Tella's negation kanji.
