Electrically, when I think of jumping my car, I visually see basically putting a stronger voltage source in parallel with a weaker voltage source (see http://imgur.com/qz8jX). However, on every car forum, they say to ground out the negative jumper on the engine-block of the dead car (see http://imgur.com/6RPYB)
All I hear is "there's gas that's formed when a battery dies, and there might be a spark that causes an explosion", that makes reasonable sense and all, but aren't we opening the circuit? Is the car's negative technically "ground"? (I reference this post: When a battery is your power source, what is ground?)
If we've defined (as described in the other post) the engine block as ground, is there actually a potential formed between the car and earth-ground?