Canon Rebel T2i Quick Start Manual page 26

Compensating for Auto Exposure
Holding the AV button on the back of
The camera and turning the dial adjusts
exposure compensation. When the dial
lines up on the middle, the optimal
exposure has been reached.
You can see that this compensation has
changed the ISO to 800. Great! But the
shutter speed remains the same.
I am shooting at 1080 30p and the
shutter speed is below twice the frame rate.
The shutter speed should be 60
(1/60 th of a second).