Flashguard is a more advanced way of guarding against attacks in One Piece: Burning Blood. The tradeoff is that you’re put right into the center of damage in order to get a huge counter opportunity.
Flashguard is performed with the basic guard button (Circle on PlayStation, “B” on Xbox One). The major difference is that you can’t block until right before the enemy’s attack connects. That means if you screw up, you’re going to get hit square in the face with a powerful attack.
One benefit to Flashguards in Burning Blood, though, is that they can be used even if you’re caught up in the middle of a combo. A normal guard, on the other hand, is impossible to use when in the middle of an onslaught. So, when used properly, it can get you out of a pinch and back in control of a fight.