* FE works everytime, just like on a regular exposure, ie if you don't lock on the subject and recompose, the camera will re-evaluate the shot for the whole frame, with flash or without flash! * FE lock with flash will hold the flash exposure based on the center spot and will hold this exposure with a TTL pre-flash, either with the built-in or the 550EX as you recompose your shot.
If you have multiple 550EX (or the Sigma 500DG Super), when you press the lens stop down button on the camera. All the units will emit a continuous modeling light pulses so you can evaluate the lighting balance/ratio (or shadows) and change them accordingly. All this is done wirelessly as well as the TTL metering. I thought this was very cool with my Minolta 9xi, and now available on this new 10D as well. 8)
* FE lock and Wireless ratio all with TTL: