If the mounted flash fired in full power, SU-4 generates repeating beeps. In either way, exposure could be incorrect. Under the auto mode, the on-camera metering system controls everything. Under the M mode while the slave flashes still synchronize with the master flash, each slave's flash output intensity can be adjusted independently from the master flash. The following is a very simple example that illustrates the use of external flashes.
More examples are available on other pages.
The left image was taken with the internal flash in the A-REC mode. This direct flash does not generate shadow because the shadow is hidden behind the subject. However, the image looks dull. The flash is placed to the left of the subject and raised above with an illumination angle of 45 degree. As you can see from the image, texture on the subject is shown much better than that of the left one.