You may have notice the engineer unscrewing a bleed valve and inserting a screwdriver to prevent it from closing prior to the start sequence. This is to stop the heated air within the cylinder from pushing the piston back down the bore as the air expands with heating. Once he's sure sufficient heat is obtained, he closes the bleed valve, primes the engine with a few squirts, the give the air lever a sharp tug to start.
These guys have over primed the engine, and the fuel accumulated in the crankcase (they are two stroke engines) begins to go up into the combustion chamber, and the whole lot runs away. A tricky situation, but they kept their bottle and starved the crankcase vent (inlet) with rags to reduce oxygen intake.