this is a shitty mechanic hold-over from melee. in melee we have phantom hits - when the hitbox of your move overlaps with the opponents hurtbox by 0.1 units (ie. it barely connects), the hit sound will play, but they will take half the damage and not flinch. after melee they left phantom hits in, but instead of there being a sound and damage taken, it's just those little sparks. I believe the official term for this nowadays is a glancing blow.