There are 2 risks, and each has a completely separate way to mitigate the risk. Unfortunately, in this thread we're clear on what I'll call risk A but there is not clarity on risk B.
Risk A: The spark ignites the hydrogen. Avoid this by ensuring that the final connection is away from a battery, especially the dead battery, by using a remote negative (unpainted metal) for that last connection. It doesn't matter a whole lot whether it's on the dead car or the live car as long as it's the last connection, although if we accept that any connection point could eventually spark (from adjustments, tripping over wires, etc.) then it's marginally better to make the dead one the remote negative. If there is no feasible remote negative on the dead car, go with the live car.
Risk B: Creating a short circuit across a battery. Avoid this by never having the clips at one end both loose while the clips at the other end are both connected. If you do have this situation, the loose ones could touch, creating a massive short circuit with no fuse inline to save you. You could argue that if your buddy is holding the end with 2 loose clips, he won't let them touch. Yeah, well what if he sneezes. And if they're on the ground, what if you kick them.
So there you have it. Everyone always thinks about (A) but forgets about (B). The "proper order" some people think is meaningless actually does solve (B). The order has nothing to do with being nice to the electrical system, since from an electrical standpoint there is no circuit until all connections are made. Think about it... if you make 1 connection at the first car, then 2 connections at the second car, then 1 connection at the first car, you cannot end up in situation (B). Live car first, dead car first, that has nothing to do with it, just remember 1-2-1.
It's only when you combine the two techniques for (A) and (B) that you end up with a definitive order. If you finish with the remote negative on the dead car, you start with the positive on the dead car to achieve 1-2-1. The combined procedure ends up being
1. Dead+
2. Live+/-
3. Dead remote-
Now there's one more small point, very small as it only matters when the vehicles are touching (via metal) and they shouldn't be anyway. In step 2, doing + before - is better. This minimizes the amount of time that you have a positive cable connected at one end and not the other. Why minimize this? Because you really don't want to drop the loose end of that cable onto any metal, or let it drag about uncontrolled while you only concentrate on the negative clip. Letting that positive clip touch metal on the second car will cause a short circuit on the first car, completed only because the cars are touching. Thus:
1. Dead+
2. Live+
3. Live-
4. Dead remote-
I think any arguments for deviating from this order are addressed in my post. Obviously the majority of this hinges on unlikely situations, but that's how being safe works.
) You SHOULD see a spark when you connect the final connection. If you don't, one or more of your connections (or ground) is poorly connected.
