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 resistent 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, Atsuhiro can capture people easily, without causing noise and before the victim can attract the attention of nearby allies. Atsuhiro is overly precautious.
This can come in handy to lay traps, which he demonstrated by throwing a marble under a police car and releasing his Quirk, causing the rock he had "compressed" to instantly recover its original size and launch the car upwards.
- Compression - Wikipedia article about the general effect caused by Atsuhiro's Quirk.