The symbol of the rune throws out sparks and so will you while having it attached on your body. The passive effect will occur in battle every time the wielder choose to attack any target with his weapon. After that any others which are meant to do the same will follow right behind. That is, if they are not faster than the bearer of course.
The Spark Rune appears during the first to games only and is well hidden inside a secret room in Scarleticia Castle basement, while a drop from the Double Head boss is assured. It can be pretty useful if you take one speedy fellow along with slow physical damage dealers. Though the rune slot in Suikoden I is precious. Suikoden II lets you combine it with a Gale Rune, thus the whole party can act at first. - Celes