Risk of explosion!
WARNING: This page does not describe the selected car, but rather 14 other vehicles, including the 2011 BMW 328xi, 2011 BMW 328i, 2011 BMW 323i, 2010 BMW 335xi, and 2010 BMW 335i. 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.