Disable master flash on 580EXii

[Pauly]

How can I disable the master flash on my 580EXii? I want to use it as a master to trigger my 430EX.

I have tried setting the flash to off but it still fires when the shutter is opened. (Press zoom button --> turn dial to OFF)

 

[Pauly]

The flash that it fires is only a pre-flash. I just figured it out by trying it with a red gel and not seeing the red.