Risk of explosion!
WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2012 BMW 328xi, 2012 BMW 328i, 2011 BMW 328xi, 2011 BMW 328i, and 2010 BMW 328xi. However, it is still accessible from the selected car via links, so may be relevant.
After the engine of the vehicle to be started has fired, first disconnect the jump starting cable between the negative pole and the earth/ground point. Then remove the starting cable from the positive poles.