From Final Fantasy Wiki

"Here we go! Go, go, go!"

—Zack

Chain Slash is a Limit Break in Crisis Core -Final Fantasy VII-. It is only usable for a short time when Zack has blank faces on his DMW slots, meaning that person has yet to be met or the summon materia hasn't obtained. Chain Slash is a place holder attack that is replaced by the other Limit Breaks once they are learned during the course of the game. The Limit Break involves dealing multiple quick, hard slashes to a single enemy, dealing major damage. No Materia is available which can power it up, and it looks very similar to Cloud's Cross Slash Limit Break.