Using a flash only to trigger slave?

I've got a quick question, if anyone knows the answer. I'm planning on shooting an event tonight with a 580EX on top of my camera as a primary flash, and a 430EX in wireless slave mode for backlighting. There will be times, though, where I'd like to use ambient light for the most part, but keep the backlight from the slave. Is there any way to have the 580EX not fire itself, but still set off the wireless slave it's connected to?

Yes. You can disable to flash pulse from the 580. Have a look through the flashes' instuction book to see the setting. By the way the 580 will continue to appear to "flash" as the first instructional pulse still needs to be sent out by the 580. E-TTL has two pulses, a short "Informational" pulse that tells the 430 how to behave, and then the second "Illuminate the subject" pulse. You are disabling that second pulse.

Thanks. I'll take a look at the manual when I'm out of school, but does anyone know the setting in question offhand?