reading and watching David Hobby from Strobist.com

editing post for removal.

ETTL is auto, but you can do FEC which can adjust the flash. for instance if you bounce the flash on the ceiling, you will need to FEC by +1 or so to compensate for the extra distance the light has the travel and the power lost do to diffusing.

E-TTL is the form of automatic flash metering used by Canon digital cameras. Yes, it's auto. You can adjust it up or down by up to 3 stops in 1/3 stop increments. This is known as flash exposure compensation (FEC). You can also leave the flash setting at O and set the FEC with the camera (+/- 2 stops).

In manual mode, .3 is 1/3 stop, .7 is 2/3 stop. 1/2 -.3 is the same as 1/4 +.7.

Multi flash mode is a specialized feature that allows multiple flashes during the same exposure. It's used to freeze a moving subject several times in a frame which is a cool effect if done right.

Automatic flash metering in various forms has been around for 40 years and can be quite valuable.

For one thing, you can't always predict what your subject distance will be. E-TTL lets you get the shot without first having to judge the distance and adjust the flash power accordingly.

Secondly, when you're bouncing flash or using a diffuser, it can be extremely difficult to guess how much power you'll need. E-TTL lets you use those techniques without trial and error.