Edward Ingold , Oct 14, 2006; 09:52 a.m.
The SU-4 mode fires the flash when the main flash fires, and quenches it when the main flash shuts off. You can adjust lighting ratios and exposure by positioning the flash (power settings on the remote flash are ineffective in SU-4 mode - they work by quenching the flash not by adjusting the intensity).
TTL flash with multiple heads is a waste of time and largely ineffective. Set the flash position and power in manual mode, and use a flash meter to test the results. The camera should be in manual exposure mode too.