Flash has evolved considerably over the years. The Yongnuo 560 is a "manual" flash. That means it flashes at a power level you manually set whenever it receives the flash signal from the hot-shoe.
The "foot" on the flash you should see a single center pin and a metal plate. The "plate" is actually the ground pin. The "rails" on the hot-shoe of your 70D are actually conductive... they provide the common ground. The center pin on the flash contacts the larger round circlular contact in the middle of the camera hot shoe. These two pins (center and ground) provide the firing signal.
But notice the 70D has 4 more pins... when you use a YN 560 flash, those pins are not used at all... the flash doesn't support them. Those are the E-TTL pins that allow the flash and camera to communicate.
Canon does not publish the E-TTL specs... 3rd party publishes reverse engineer the system to build compatible flashes. But what this really means is that there are different degrees of compatibility. E-TTL can do a lot... very few 3rd party manufacturers actually support everything possible If you want "full" control of the flash through the on-camera menu, then it's best to just buy a Canon brand flash.
The basic E-TTL function is to support automatic flash exposure setting. When you take a flash exposure, the flash technically fires twice, but it happens so quickly that most people presume it was just a single flash. It meters the scene before firing flash, fires a "pre-flash" at very low power (usually 1/32nd power) while metering the scene again... and compares what happened to the no-flash vs. flash metering samples. This helps the camera determine how much power should be used when the shutter is open for the exposure. Finally the shutter opens and the flash fires using that pre-determined amount of power. I've simplified the explanation because there are lots of clever nuances of how it works.
It turns out there are many more features of E-TTL systems.... the ability to do "hi speed sync" (use flash when the camera shutter speed is higher than the maximum camera "sync" speed), 2nd curtain shutter, multi-strobic flash, assignment of flash to flash-groups, flash ratios, flash exposure compensation, etc.
Yongnuo makes some versions of their flashes that include some TTL functionality, but the 560 is a basic unit. It's advantage is it's low-cost, but you do have to manually control everything. You control flash exposure by dialing the power level up or down, opening or closing the lens aperture, or changing the distance between flash and subject.
EVEN with flash that supports E-TTL, the ability to use on-camera menus to control flash settings is a fairly advanced use of E-TTL. Only more recent Canon flashes support that.
Tim Campbell
5D III, 5D IV, 60Da
