The warning not to sit or lie on top of your heating pad is required by UL Standards, the guidelines used by the industry for testing and approval. It must be included on any product that is currently tested and approved for sale in the US by UL, ETL, or any other Certified Testing Lab.
The Thermophore® is constructed with heater wires in a fabric foundation. Folding or lying on top of the unit traps and intensifies the heat at "pressure points." The Thermophore® is also able to trap more moisture from the atmosphere when it is not insulated between your body and the cushioning of your furniture or bed. In addition, our units reach a much higher temperature than ordinary heating pads and it would be possible to damage your skin (or your furniture) by trapping and intensifying this heat.
Compressing the Thermophore® when it has become folded or bunched under your body can also damage the special metal strips inside, and they serve several functions. They hold the heater wires in place throughout the entire area of the Thermophore®, help distribute the heat and retain the heat level when the switch is shut off, increase the production of moisture, and make the heating unit mold against the contours of your body to make good contact for even transfer of heat. Lying or sitting on a Thermophore® will often mangle these strips, reducing there effectiveness.