They are described in the books as small, round, and metallic. They hold kerosene, naphtha, or sugar. When they are activated, the fuel burns and the sympathist has about five minutes of boosted heat until it dies out.
In The Wise Man's Fear, Devi uses one to beat Kvothe in a duel. During the duel Kvothe reveals that Kilvin refers to them as "self-contained exothermic accelerators". He goes on to say they are messy, dangerous, and prone to breaking, and that they also need to be dismantled and cleaned between uses.