Miniature Jump Starter

I think the key to how "well" these work is that they have some level of smarts inside of them.

Most of the ones I have seen take a bit of warm up time before they are ready to actually jump the car. My guess is that the box is getting some baseline for the battery charge. As soon as it sees the voltage tank (when to go to turn the engine over) they deliver their "jump". The spike they provide has been compared to the discharge of energy observed when you short a out a battery.