All remote flashes need a way to communicate with the camera. They can be wired or wireless. The wireless system can be optical or radio.
The main thing is that there is nothing built into the 6D to control a flash... so you need something to do that job.
The 430EX II can be triggered via optical signal as a remote "slave" flash (the 430EX II cannot function as a "master" (aka "commander") mode.)
The Canon method to control the 430EX II is to use either:
a) A "Speedlite Transmitter ST-E2"
b) Another speedlite capable of working in "master" mode (and there's a long list... the 600EX / 600EX-RT, the 580EX II, and oddly enough the 90EX is actually capable of performing as a "master").
I would recommend either the 600EX-RT (which would lay the foundation for additional 600EX-RT units in the future which would give you radio triggering) or find a 580EX II. I would probably pass on the 90EX and favor an ST-E2 if _all_ you want is a way to trigger the remote flash and don't actually want a flash. The 90EX isn't very powerful so it's not especially suitable as a stand-alone flash unless you're just using it as a "fill" flash for close subjects.)
There are also lots of 3rd party triggers but the vast majority of these wont let you use E-TTL (they'd be 'manual' only) and the units that support E-TTL will be as expensive as just buying a high-end flash that work as a commander.
Tim Campbell
5D III, 5D IV, 60Da
