Hey guys,
I was watching something online about flashes a little while ago and the person in the video mentioned that you could trigger most external flashes to go off with the light of an on-camera flash hitting the external flash's sensor. Is this possible to do with the 430ex and how can I do it, if it is? Thanks.

You need to use canon flashes to do this, and they only work with the IR trigger, not a visible light trigger (sorry, the correct term is slave)

The 430 EX can be triggered by infrared pulses from an ST E2 or one of the 580 EX models. Supposedly it can also be triggered by a Sigma EF-530 DG Super E-TTL II flash (Canon mount), but I have read reviews that the Sigma sometimes has trouble triggering the other flashes.

Optical slave flashes can be triggered by the on camera flash, but there are two kinds of slaves, the old fashioned kind that are simply referred to as "optical slaves" and the more modern "digital slaves". Digital cameras fire one or more pre-flashes before opening the shutter and firing the actual flash pulse that illuminates the subject. The optical slaves will fire on the first pulse they see, which will be the pre-flash, and not be able to fire again the couple of milliseconds later when the "real" flash fires. This all happens too quickly for the human eye to follow, and it looks like everything fired fine, but your picture will be underexposed because the slave fired while the shutter was still closed.

Digital slaves are programmed to ignore the preflash and wait for the main flash before they fire.

The slave flashes do not communicate back to the camera, so you lose E-TTL and will have to do it the old-fashioned way, adjusting the flash output, flash-to-subject distance, aperture, and ISO to get the desired exposure.