Heat projection was a natural feature of Scarran biology which they used as a weapon and a method of torture. Heat is generated by a gland in their upper chest. Removal of this gland by surgery not only removes their ability to project heat but also weakens them physically. This is shown in Naj Gil in the episode "Fractures".
The heat from inside their bodies is projected through their hands and through their mouths. This form of attack can kill a Sebacean in seconds because of their heat delirium, but is just as effective against other species. Victims of these attacks can be identified by bubbled skin and a face twisted in a final grimace of pain.
Scarrans use heat blasts of lower intensity to torture prisoners for information or for their amusement. Heat projection is used to frighten, weaken, cause pain, and otherwise attack the resolve of the victim. Then they probe their mind with some form of mind reading. Sometimes other tools are used to assist the process. The heat projection can be beaten as is shown in the "We're So Screwed" trilogy.
The ability to project heat is lost in cold environments. The heat gland is needed to maintain body temperature for survival because heat is such an important part of their physiology.