What is the safest way to hook up car battery jump cables and why? I know that when both connecting and disconnecting battery jumper cables between two car batteries there is a prescribed safe way to do it. I can never remember the correct way because I can’t understand the reasoning behind the prescribed sequence. Is there an easy explanation / (chemical?) principle that can make this procedure (the sequence of both connecting &amp; disconnecting the four posts) more logical and therefore easier to remember? 

My confusion is that at some point you must make a connection that causes sparks (which risks battery explosion) because it completes the circuit. Completing the circuit can’t be avoided, so why does it matter which post you are connecting to when you complete the circuit?
Many recommended procedures such as this one call for connecting (+) to (+), then (-) on the "good" battery to a metal component in the "dead" car engine.  The negative terminal of the battery is grounded to the metal components, so we may think of the last connection as (-) on the dead car.  If a spark were to form when the circuit is established on the last connection, it would occur at the metal component and not at the terminal where it could interact with electrolytically generated hydrogen or oxygen gas.