It works but because it passes a current through what you are soldering it is not recommended for printed circuit boards. My impression is it is more gimmick than useful tool.

As posted above, it heats the conductor you are soldering and not really the joint. With that in mind, it should never be used on any static sensitive electronic components (most all) and will most likely create a "cold solder joint" on anything you use it for.

Soldering is done with both sides of the joint getting to, on average, to 750 degrees, and allowing the solder to flow between the two of them to create the junction.

Similar units were helpful soldering radio components etc back in the 1950s and early '60s when an ordinary resistor was about the size of a cigarette butt with two inch leads sticking out each end. Some types used the carbon rods out of old D and C batteries ground to shape for soldering tips, talk about early recycling! Anyway, as soon as solid state components like transistors and diodes showed up, the scrap dealers got most of them.

It's a cheesy form of resistance soldering. Resistance soldering works well in certain applications. It requires specialized equipment which costs a whole bunch more than the stuff sold as "cold heat."