I’m seeing more ads for gadgets that will help you jump start one car from another. They plug into the cigarette lighter outlet in both cars. I gather that they’re 12 v outlets, like the batteries, but somehow, that doesn’t seem like enough juice to jump a car. Not only that, but all those big copper cables and the danger of dealing with batteries makes me wonder how it can all be safely powered down to a little cord dangling out of the cigarette lighter. Being a pinhead when it comes to the way electricity works, I’d like some straight dope. 'Sup?
It takes a long time to squeeze enough electricity through those little wires to recharge a battery enough to turn over. Do any of these gadgets claim to work exactly like jumper cables (plug it in and you can immediately start the dead car)?
My best advise, for whatever it’s worth…don’t waste your money. You said it yourself. “that doesn’t seem like enough juice to jump a car.” It isn’t. There’s a reason jumper cables are made from such heavy wire. 
Probably not, and that may be the way they do it. Dribble enough juice from one car to another, over a good bit of time, to recharge the dead battery.
Sometimes they work just fine. It depends on how low the charge is in the recipient battery. The lighter circuits on both cars are fused, so’s there’s not really a safety concern. Their advantage is in being easier to carry/store and simpler to use than jumper cables. The disadavantage, as noted above, is that if the recipient battery is significantly low they’re not going to get the car started. While they could get the job done eventually, who wants to have the donor car idling in place for hours just for that?