The sushumna is the line the kundalini travels up the spine. It is very small in diameter. Tibetan monks would be tested for initiation by stripping down naked, wrapping themselves in a wet sheet, and spending the night in the freezing mountains, sitting in the snow. In the morning, if the initiate passed the test, the sheet would be bone dry and several feet of snow around the monk would be melted.
Expanding the sushumna is used to raise body temperature and create heat. This power can also be applied to pyrokinesis (setting objects on fire with the power of the mind).
While in a trance, visualize the sushumna expanding to the spine in either a red hot or white hot glowing light. Focus on this for several minutes and from there, expand it beyond the spine, again, focus for several minutes and then expand it 2 inches beyond the spine. Keep expanding until you are several feet or more beyond your physical body.
This meditation produces a lot of heat, especially when done regularly. When practiced on a frequent basis, one can expect a rise in body temperature.