Complex sentence: The back-burn was created by dropping "ping pong balls" from a helicopter; these are spheres the size of a ping pong ball filled with a flammable substance that ignites as they impact the ground.
Split into simpler sentences: The back-burn was created by dropping "ping pong balls" from a helicopter. The balls are small spheres the size of a ping pong and filled with flammable substance that ignites as they impact the ground.