Confession allows Shin to force anyone he asks a question to answer truthfully.
With this Quirk, Shin can detect the true thoughts, secrets, intentions and feelings of allies and enemies alike, allowing for information gathering, psychological torture and betrayal prevention. The usefulness of this power has apparently made Overhaul value Shin the most out of the Eight Expendables.
It's worth noting, however, that this Quirk doesn't guarantee the prevention of betrayal, as people with unpredictable or unstable natures (such as Himiko Toga and Jin Bubaigawara) can simply decide to change their minds in regards to their loyalties on a whim.
Confession has little offensive capability, so Shin has to rely on other methods to fight directly.
- Confession - Wikipedia article about the effect of Shin's Quirk.