Talk:FAQ/@comment-78.241.130.181-20200427215107/@comment-5849797-20200428000443

You need at least 10 FP with the character in question, and the both of you need to be at least level 30. Once both of those conditions are met, the screen might flash randomly during a battle with a message box saying that "[Player] and their partner are in perfect sync!" Once this happens, the skill "Sync Your Breathing!" will appear at the bottom of your skill list. Using it will deplete 100 SP from both the player character and your partner, and if you can both make a move (you don't get afflicted with BlowBack, Fear, or any other status that stops you from attacking), you'll unleash a powerful Combination Attack.

It's likely to do anywhere from 500-1500 damage, depending on your characters and what enemy you're fighting. Every character and playable Stand has a unique combination involving them using their powers in tandem, but it's random if it'll be that or just a generic combination attack that involves the both of you attacking. Damage is unaffected, though.