That's a very old flash. The center pin and ground plate on the flash (or if the ground plate is plastic, there will be a little metal contact on one side which contacts the metal rail on the hot-shoe) are the key pins... when you close that circuit, the flash will fire and will release a trigger voltage into the camera.
That voltage needs to be 6v or LESS. Damage could occur if it is greater (and there are a lot of old flashes for which that voltage is greater.)
You can find a list of trigger voltages as measured and submitted by individuals (hopefully accurately -- but this is the Internet) here: http://www.botzilla.com/photo/strobeVolts.html
They don't list your Promaster FTD 5600 but they do list other Promaster FTD series flashes and of those listed, they all appear to be in the safe range. If you own a multi-meter, that page has instructions on how you can test your flash before attaching it to your nice new camera.
Another thing to point out is that since that's a very old flash, it can only be used manually (and it's manual function isn't even all that great because you can't dial the specific power level output that you want) or via the Thyristor circuit. Thyristors measure the light returning after bouncing off your subject. They highly suceptible to be wrong depending on how large or how reflective your subject is. Modern E-TTL II flashes are vastly improved and I'd strongly consider an upgrade. Take a look at a Speedlite 430EX II -- which would be a great external speedlite for use on a T3i.
Tim Campbell
5D III, 5D IV, 60Da
