OK, in that scenario what is supposed to happen is the flash would play no part in the exposure, you should, in theory, end up with a one stop under exposed wall.
However in my experience given that scenario, that is not what you do end up with, but in fairness that isn't what E-TTL is set up to do either, the assumption is that you have the flash on to light something so it tries its hardest to do just that! In my experience you end up with a slightly under exposed wall, but not one stop under, and the flash is clearly visible.
There are many situations where E-TTL will struggle, and not having a subject in the frame is one of them. Try your scenario with a more realistic set up, put something in the frame closer to give the flash metering something to work with.
Thank you very much for you input.
I will play a bit more with my speedlite tonight.
After that being said, I will note what you said, and see if I can understand my flash better. and get the most of it.
The main reason behind my question is: I wanted to full used my 90ex. even for just a bit of fill.
On Six burst shot:
On ETTL, Flash fire alternately.
On M mode (flash), 1/32, flash fire on every shot.
So last night, I was trying to get the balance right, use ETTL
and at the same time, flash must fire on every shot.
But even i set camera exposure to +1 and FEC to -3.
Flash fires strongly resulting to not fire on the next shot..