Big Fist allows Itsuka to enlarge one or both of her hands to enormous sizes.
Itsuka's hands become gigantic (around three times bigger than her own body) almost immediately. Her physical strength also increases in tandem, giving her enough power to easily crush tungsten shields, knock out people with a few strikes and carry heavy objects with her. Itsuka herself does not seem affected by the increased weight of her gigantified hands.
Big Fist is used for close-range combat, as Itsuka has enough power to break through most defenses. The sudden expansion of her hands can be used to surprise foes, assuming they're unaware of how her Quirk functions, or increase the power of her attacks through the momentum of the expansion.
The enlarged hands are not only useful for combat, but can also be used as a method for transporting others and restraining opponents as well, making it very convenient for rescues and criminal escorting. Someone wrapped by Itsuka's giant hands will become protected from certain hazards, such as poisonous gas, albeit just temporarily.
By flapping her enlarged fists around, Itsuka is capable of generating wind, letting her counter gas traps.