Compress allows Atsuhiro to shrink down anything inside a spherical area into a small marble without actually damaging it.
To activate his Quirk, Atsuhiro first needs to touch the target with his hand. This will result in the target getting shrunk down into a minuscule marble-like object. Compress will affect anything that is found within a specific range of space around the intended target, including, for example, fire or pieces of the ground (the exception being Atsuhiro's own arm). The weight of the compressed objects is reduced considerably, making it possible to hide or carry them around. Atsuhiro is able to cancel his Quirk's effects on a single marble by wishing it or all his marbles at the same time by snapping his fingers, which causes the target(s) to turn back to their normal size alongside everything that was captured with them.
When used on a person, Compress effectively entraps them, terminating the fight in an instant and letting Atsuhiro abduct them easily. This is particularly useful against opponents who are resistant to certain forms of attack, such as Snatch. If Atsuhiro focuses on an area smaller than the target's whole body, he is able to compress only said area while leaving the rest of the target's physique unaffected, mutilating them in the process.
Atsuhiro, being a former stage magician, employs several tricks in combination with his Quirk in order to mislead his opponents. Thanks to his amazing stealth and agility, he can capture people within compressed marbles without causing noise and before the victim alerts nearby allies. For the same reasons, Compress is very useful for transporting downed allies.
Atsuhiro is overly precautious with his captives, utilizing his illusionist skills to sneakily create decoy marbles and swap them around with the ones holding the actual targets, thus deceiving anybody who attempts to steal from him.
Compress comes in handy when it comes to laying traps. If Atsuhiro has some marbles containing compressed objects, it's possible for him to use those as a way to obstruct the path of enemies, after throwing said marbles at their direction and making them grow back to their normal size. Thanks to Atsuhiro's creativity and ingenuity, he is able to take advantage of the cancelling property of his Quirk to affect the environment around him, like placing a marble holding a compressed large rock below a car, and then launch the vehicle into mid-air through the marble's uncompression process.
Compress also works as a way to dig through solid objects, making Atsuhiro useful for escaping enclosed areas as well as breaking into them.
- Compression - Wikipedia article about the general effect caused by Atsuhiro's Quirk.