This ability increases their attack speed by 5% the more time they spend attacking, additional units together will build up more percentage, to a maximum of 25%.
So for instance if you summon 1 Sylph to attack your enemies that slyph will gain 5% attack speed bonus. However use more than one Slyph the attack speed bonus will be multipled, up to the maximum of 5% at 5 stacks.
Their accuracy also slightly increases, in order to make them more potent in dealing damage.
In addition, slyphs have a chance to parry one attack by fading into the invisible, this can only occur about evey 5 hits. But dont forget that as the quicker they attack the more often might parry.