All EOS cameras have auto fill flash algorithms built in. If you use your flash in any mode in bright light it will automatically reduce flash output for a natural look. P mode defaults to main light in dim light (below EV 10) whereas Av, Tv and M (if you follow the meter!) always expose for the ambient and use fill-in flash.
It's also possible to defeat auto fill flash reduction via CF 14.1 and dial in your own reduction (you don't want to dial on top of auto fill flash reduction as its hard to predict what the camera will do).
So basically you can point 'n shoot 'n get great fill flash or set CF 14.1 and roll your own.
You might want to study these "EOS Flash Bibles" for details:http://photonotes.org/articles/eos-flash/http://web.canon.jp/Imaging/flashwork/index.html