Thedude1,
this is type dependent actually. A Crossfeed will allow feeding an engine with the contents of the other engines dedicated tank - this seems to be the more common option (used on F70/100 and apparently many other jets), while a Crossfill system will use some pumps (on the DH8, the auxiliary fuel pumps that are run during approach/takeoff) to pump fuel from one tank to another; the engines will always be fed from their own tanks.
Now in case of an engine failure, the Crossfeed on the Fokker was the easiest thing imaginable to use - just open the valve and leave everything else as it was; no need to worry about imbalance any more. The Crossfill on the other hand seems designed to provide the crew with more workload (like many other little details on the Dash) - You will need to manually transfer fuel from the dead engines tank to the live one every so often, minding the imbalance Yourself.