It's not a buff or a debuff. As for how it works, say you have a 100% chance to stun on an L1, if it glances, that hit has -100% ability accuracy, so it instead will not stun.
If whatever move you use has a buff/debuff activate while you hit the enemy, and it glances, that buff/debuff will not activate on that hit.
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If whatever move you use has a buff/debuff activate while you hit the enemy, and it glances, that buff/debuff will not activate on that hit.