Electricity ALWAYS takes the path of least resistance. If you look at the diagram you posted, there's a note between the two "load cables" that states,
(EQUAL LENGTH CABLES). An extra foot or two of cable on one lead will change the resistance in the cables and can affect the electrical flow between the batteries so one provides more flow (discharges faster) then the other. This becomes important when recharging the batteries. Think of it along the lines of putting cups of water (electricity) into a bowl (battery). If you add water equally, and the bowls have different water levels, one will overflow before the other is full. It's the same with batteries: fill them equally, empty them equally and they will "balance" longer without the stress of unequal discharge/recharge cycles.
What you're doing by connecting the + terminal on one battery and the - terminal on the other battery, is forcing the electricity to be drawn from both batteries "equally". If both of the load cables were connected to the same battery, the "path of least resistance" would be through the top battery only and the bottom battery would only supply electrons (flow electricity) at a reduced rate consistent with the amount of resistance in the cables and connections between the two batteries.
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John
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS
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