In addition to the normal vaccuum insulation layer, it has a layer that does a (constant-temperature) phase change at ~140F. If your drink is hotter than this, energy flows out of the drink, cooling it and liquifying the layer. If you drink is cooler, energy flows into the drink, solidifying the layer.
grep for technogeek [here](https://joeveo.com/pages/the-temperfect-mug)