Complex sentence: Flash powder derives max efficiency and power by gas/heat expansion which is the underlining reason all commercial based salute fireworks are filled 2/3 with flash thus leaving 1/3 air space for gas to expand making for a much more efficient and violent explosion.
Split into simpler sentences: Flash powder derives max efficiency and power by gas/heat expansion which is the underlining reason all commercial salute fireworks are only filled to 2/3. Leaving 1/3 air space leaves room for gas to expand making for a much more efficient and violent explosion.