Please help me understand what I did from with my Speedlite

Hi,

Yesterday I shot a graduation party. I used my T2i and a friends Canon 550EX Speedlite. I had spent some time the prior months familiarizing myself with the 550EX and thought I was ready…

It was a very sunny day and the guests traveled inside and outside a large tent. The 550EX was set on E-TTL exclusively. Using the 550EX inside the tent with a diffuser bounced off the white ceiling worked wonderfully. However for the outside shots I was hoping that the 550Ex would leverage the E-TTL feature to simply add a small amount of light to balance the backlight photos (I never shot into the sun, it was just a real sunny day). I failed. The people were 1-stop underexposed and the background 1-stop overexposed so I had to spend a ton of time in post.

I clearly misunderstood the function of E-TTL and/or don’t understand how to leverage the 550EX. Can anyone provide some guidance? Any references or thoughts?

Thanks in advance.

Best,