When is "off" off?

If I attach a motor to a battery box and twist the motor arbor I get resistance
EVEN if the battery box is switched off.  This means that the current,generated
by the motor, is getting through, correct? Is the current just bridging the
contacts or is it going through to the batteries?  Is there a 'drain' condition
if motors are left plugged into battery boxes or RCXs?  I know the gurus out
there can give me an answer.  Thanks.