TV coverage of the massive Yosemite Rim fire typically shows the Hercules planes and helicopters which are dropping retardants on the flames. Less is talked about how controlled fires are started to burn brush to divert the direction of the big flames.
This site explains
“One form of aerial ignition uses a plastic sphere dispenser (PSD) machine, commonly called the ‘Ping-Pong Ball System.’ The PSD, generally mounted in a helicopter, feeds Ping Pong-like balls through a chute leading out of the helicopter. The balls, which contain a chemical oxidizing agent, are injected with a water-glycol solution (antifreeze) as they are shot out of the PSD. The chemicals react thermally and ignite in 25-30 seconds out of the chute as they fall to the ground.”