The experiment started with the intense heating of potassium nitrate (KNO3). Upon heating, potassium nitrate first melts, then starts to decompose. Oxygen evolves from the molten salt in form of small bubbles:
2KNO3(l) → 2KNO2(l) + O2(g)
A piece of coal (charcoal) was thrown into the molten potassium nitrate, where it started to bounce intensively as it burned in the emitted oxygen:
C(s) + O2(g) → CO2(g)
NB! The glowing piece of coal is likely to jump out of the test tube!