It is kind of like everything else that goes into a story. If you pull it out of nowhere and spring it on your readers, they will feel cheated.
Your readers need to have the feeling that this unexplained ability is part of the story, and not merely something that the author pulled out of the hat to get the characters out of a sticky spot.
Say you have a character in a story about truckers and trucking. Lots of scenes about loading trucks and driving cross country. Lots of stuff about truckers who meet up in truck stops because their routes cross, just all kinds of ordinary things that happen to truck drivers. This character drives up to a fenced in building to deliver something, and there's no one there to open the gates. The only place to turn around is inside the gates, and backing out isn't an option because the road is five miles of switchbacks. The driver gets out, picks up the truck, carries it back up to the highway, gets in and drives off.
That's going to really cheese off your readers. They thought they were reading about some average guy doing normal things, and the author has casually thrown superman at them just to avoid fixing the story so that the trucker can get on with his work in a normal way.
Your story has to at least hint at the existence of special abilities. It has to at least hint at the character having special abilities. Just dumping them on your readers cold looks lazy - like the author couldn't be bothered to come up with a good solution and just punted an "eh, magic, good enough" at the readers.
You don't have to explain how such special abilities work. You just have to give them a plausible reason to exist within the story. There needs to be some pattern that your readers can see that connects the special ability to the story.
Take Major Joachim Steuben from David Drake's Hammer's Slammers novels.
The Slammers are a mercenary outfit with members drawn from all over the human universe - hundred, if not thousands of planets.
The members count themselves as the best of the best at what they do - winning battles at whatever cost it takes.
Steuben is the best of the best as a gunman. Fastest shot with a handgun, best shot with a handgun.
In one story, an important character is killed by a shot fired from a handgun.
There's no way anyone could have gotten close enough to intentionally shoot the guy - fenced enclave, guards, etc.
At the time of the shooting, there was a battle going on a good mile away - Major Steuben was in that battle, and was known to have not gone to the enclave where this important character was shot.
There's an investigation with the conclusion that the guy was watching the battle from within in the enclave and was hit by a stray round from the fight because "nobody could intentionally hit a human sized target with a handgun from that range" - at which point, Steuben smiles. The implication being, of course, that Steuben had shot the guy from a mile away with a handgun.
Steuben is nearly magically good with a gun. That was well established in the stories. This one incident is accepted as Steuben pushing his already existing abilities to the utmost - he did it, you are not quite sure how, but you know he's good. The story doesn't say how - maybe he used a rifle firing handgun catridges or some other trick. All the reader knows is that Steuben is good, and that he will find a way to succeed.
It works because it fits established things within the story rather than popping up out of thin air.