What is the technique for Vacuum Flask Cooking?
It's a slow-cooking technique involving a thermal cooker, or vacuum flask, The pot/flask and contents are heated to cooking temperature, and then sealed in the flask. The flask more or less eliminates heat loss, so the food remains at cooking temperature for a long time, and slow-cooks without continued heating.

See this Wikipedia article for more info:
http://en.wikipedia.org/wiki/Vacuum_flask_cooking