My understanding is that with most BEC's you just tap your battery power and plug the BEC into a channel and you are done, at least that's what everyone at my LHS says to do. The BEC will take most of the load off the ESC at this point, but I imagine that the ESC may still supply some power to the circuit. If you got a higher voltage BEC, like a 6v, then that would almost eliminate the ESC from supplying power because it's output voltage would be less than what's on the line (current can't flow out of the ESC BEC) and the external BEC would be feeding everything. I suppose you could remove the 5v line from the ESC with no harm done. I would follow the directions that come with the BEC.