After asking a question, Confession allows the user to force the victim to answer truthfully.
With this Quirk, the user can learn the true thoughts, secrets, intentions, and feelings of allies and enemies alike, allowing for information gathering, psychological torture, and prevention of betrayal. The usefulness of this power has apparently made Overhaul value Shin the most out of all the members of the Eight Bullets.
It's worth noting, however, that this Quirk is very literal. When Himiko Toga and Jin Bubaigawara were asked if they were told to betray Overhaul, they said no, as they had simply been told to remember that the Shie Hassaikai were beneath them. Their later betrayal of the yakuza organization was due to their own desires to avenge Magne.
Confession doesn't have any offensive capabilities, so the user has to rely on other methods to fight directly. Also, given Confession requires both the user and the target to be oral, this Quirk is ineffective if either of them is incapable of speaking.
Shin uses this Quirk to learn of information that Overhaul wanted to know, as well as checking to make sure that no one had any intention of betraying the Shie Hassaikai.
In battle, he plays on people's emotions by forcing them to admit things that they are not proud of.
- Confession - Wikipedia article about the effect of Shin's Quirk.