Sainsmart L293d motorshield ext. power

I have this shield, and when I hook it up to my uno and USB, the ext power terminal registers voltage! I can power a dc motor from this! I don't think that is supposed to happen. According to adafruit I believe it says I can hook up power to this terminal, but it gives a warning about hooking this up correctly. Has any one tried hooking up ext power to this shield? Can any one help me understand this? I do not want to hook up power to a terminal if it is already hot!

Supply the power either through the Arduino Board or the on the power header of the Adafruit Motor Shield. Only if the Arduino Board and the Shield use a seperate power supply, the jumper must be removed.

This came from the adafruit section of the arduino reference section. What jumper is it talking about?

At least can someone tell me if I can hook up ext. power to the terminal if it reads hot when the arduino Jack is powered up?.