A glass rod is dipped first into the concentrated sulfuric(VI) acid and then into solid potassium manganate(VII). The demonstrator then touches a small piece of cotton wool with the end of the glass rod that has the mixture on it. The sulphuric acid converts potassium manganate VII into the unstable and highly oxidising manganese VII oxide. This causes the cotton wool immediately to burst into flame.
The reaction can be made a little more spectacular if the cotton wool is first soaked in ethanol.