The Thermaltronics Curie point soldering irons, operate by using a high frequency RF wave delivered into an induction coil, which is wrapped around the soldering iron tip. This tip is made from a special alloy designed to have a specific curie point and the coil and tip are electromagnetically coupled just like the secondary of a transformer.
The skin effect causes the high frequency current to remain in the outer surface of the soldering iron tip, rapidly heating it to the curie point of the tip. At this curie point, the tip loses its magnetic properties and the tip and coil are decoupled. With the tip and coil decoupled, power is no longer being delivered to the tip, and thus, it does not continue to heat up.
However, as you begin to solder with the iron, the temperature drops below the curie point of the tip, allowing the tip and coil to recouple and once again power is delivered to the tip causing it to heat to the curie point. This process is repeated very rapidly providing a very stable soldering temperature.
The curie point of the soldering irons tip is effectively what sets the temperature. Since the tip loses its magnetic properties at this point, the heater can't continue to heat the tip once the point has been reached/ exceeded. This produces an incredibly precise level of regulation that in theory means the soldering iron cannot overshoot the rating of the tip. 

M Series Tips for use with the TMT-9000S and Metcal MX Soldering Equipment
K, S & P Series Tips for use with the the TMT-2000S Series and Metcal MFR System