The heating elements of the plumbing irons work by converting electricity into heat using electrical resistance.
Resistance to an electrical flow can be created by a coiled wire ( inside the heating elements). When the electrical charge encounters resistance (the coiled wire), it makes it very difficult for the electricity to pass through and this produces heat.
When the plumbing irons are switched on, and the electricity meets the resistance (coiled wire) in the heating elements it creates heat.
This is then transferred into the clamping heads (through the shims) and conducted on to the copper piping, which then heats it up to solder temperature.