Please do not try those ridiculous 12-volt power plug to 12-volt power plug cables because the wires in the dash are too small and many cars (Leaf included) must be on for the power socket to be energized. If you try it, there is a good chance of blowing fuses, melting wires, or damaging power distribution modules and very little chance of actually starting the other car.
I've never actually used one, but carried one for years. Are you saying this from experience (yours or others) or are you guessing based on the wire gauge? I ask that because the instructions clearly say to leave it connected for 10 minutes or more, until the light on it goes green. They are supposed to work like a faster trickle charger more than jumper cable. That would also warm the battery, which is why the AA battery powered lighter units that were sold for a few years also sometimes work. I think they are fused, although I'd have to check again. I agree that they are of limited use if only because both cars must have active 12 volt power ports for them to work. The downside to the lithium battery jumpstarters, of course, is that there is always a small chance that they will burst into flames and destroy the car and anything nearby. :shock: