The use of heat to control unwanted plant growth has been around for many years and was certainly in use before the widespread use of inorganic chemicals. A common misconception is that the plant has to be visibly scorched before it will suffer. The fact is, a plant leaf only has to be briefly exposed to a temperature of around 80°C to suffer terminal damage.
This exposure to severe heat bursts the cells in the plant leaf. This in turn renders the plant incapable of converting light energy in to chemical energy (Photosynthesis) effectively causing starvation.
Further thought and research on this subject will reveal this method is most effective on young, fleshy, hungry plants rather than old established woody plants, which may have sufficient amounts of stored energy in their stems and roots to overcome temporary food shortages.