"Wood Gas" is the term used to refer to the collective combustible gasses released from wood when burned in a low-oxygen atmosphere. Most commonly the fuel in use is wood, but other substances can be used, such as corn cobs, coal, cow manure, peach-pits, and other well-dried biomasses.
Instead of burning hot and bright like a campfire, the fuel in the gasifier is choked and may be better equated to smoldering than burning. The lack of oxygen and slow burn rate encourages the fuel to release gasses like Hydrogen, Methane, and Carbon Monoxide in the smoke. The woodsmoke is pulled by the natural vaccuum of the truck's engine through a condenser, which also filters and cools the gasses before they reach the engine.
Before the woodgas line reaches the engine, it is mixed with outside air. This mixture is closely regulated by the driver to maintain an optimum ratio of combustible gasses to clean air, as a mixture too rich or too lean will in either case stall the engine. After it is mixed, the gas/air combination enters the engine and is ignited by the sparkplugs just as gasoline vapors would be, thus running the engine with cleaner, cheaper woodgas.