I hope someone will correct me if I'm wrong but, here goes:
If you use the flash in TTL mode: the camera shutter opens; the flash fires, the camera sensor decides when enough light has entered, it then tells the flash 'enough' and the flash stops.
All of this is coded within micro-flashes within the flash pulse (that our eyes don't notice). This is why you must have on camera flash (either a speed-light which is TTL master capable or the on board flash) for it to work.
When the flash is on camera, the camera measures the distance between camera and flash (focus) and tells the flash how strongly it ought to fire (all part of the pre-flash you do often notice when the camera is metring when you half press. The further the distance = the stronger the flash.
Am I right so far?
What I don't fully get, and where my off camera trials have been stuffed, is: when the flash is off camera and say 5 metres from the subject but the camera is 10 metres from the subject. It seems to me that the camera assumes the off camera flash is 10 metres (when it is only 5) and fires a far stronger pulse than it ought to. Being half the distance, this would be 4x the required strength???
The results I've had so far suggest this is the case. (I find it easy to set the off-camera flash manually).
Now, I do realize that the sensor will see that blast then, once the requisit light has entered, tell the flash to shut off. But, if you are trying to use off camera flash to side light (for eg), won't this blast of side light throw off the metering for the on camera flash (and probably drown it out).
God, getting my head around TTL flash hurts