Tummo is explained pretty well in “The Bliss of Inner Fire” [1]. The practices from Vajrayana Buddhism are very different than what you’d find in a Theravada vipassana practice. It relies on lots of visualizations and breath work.

[1] https://www.amazon.com/Bliss-Inner-Fire-Practice-Naropa/dp/0...

You might start here:

http://en.wikipedia.org/wiki/Tummo#Scientific_investigation

If you're interested in how to perform the actual meditation, this is a great book: http://www.amazon.com/Bliss-Inner-Fire-Practice-Naropa/dp/08...