4 + S + H
Slash Back is a feature in Accent Core which allows any character to "parry" any form of attack(Projectile, Overdrive, Normals) by pressing 4 or 1+S+H. Attempting to Slash Back looks like a gold FD ring around the character for a brief moment. Successfully Slashing Back will cause the character to have a gold aura to flash around them, and they will recover insanely fast. You will recover fast enough that basically any poke in the game you will wind up on advantage on if it is not stringed.
Slashing Back costs SLIGHT tension to do. You cannot "hold" Slashback like you could FD. It needs to be tapped. Successfully Slashing Back will not only give you the parry, but it will net you a small amount of tension on success. You can Slash Back multi-hit moves as well as gatling chains. You must Slash back AS the active frame becomes active. AKA you do not want to do it "before" the hit hits, like with Instant Blocking. You want to do it just as it hits you like a Red Parry in 3S or something.
You have about a 2-3 frame window to activate a Slash Back properly. You also cannot Slash Back for the first 10 frames after you have risen from the ground. You must Slash Back the correct direction (high or low) for the parry to take place (IE you can't 1SH a Dust). You can do either 4SH or 1SH for mid hits that do not hit high or low.
If missed, you are left vulnerable for approximately 30 frames during which time you can block nothing.If you whiff a Slash Back, you can still try with another Slash Back again, but your guardstun will then be 4 frames instead of 2. On that note, Slash Back usually has a guardstun of 2 frames if you're on the ground, and 4 frames if you're in the air, regardless of attack level. You take no block damage and your guard gauge does not increase.
Doing a Slash Back during a dash will cause you to stop dead in your tracks, similar to FD stops.
Slash Back is no more difficult than instant blocking NORMAL attack strings. It can be horribly punished with delayed attacks though so it's risky to use. So far it doesn't seem overpowered, just a good defensive option similar to Instant Blocking, just less safe and more rewarding.