When a nitro RC is "on," its engine is running. To keep the vehicle from automatically moving, a "clutch bell" is attached to the output shaft. On the inside is a centrifugal clutch assembly that only engages when the engine is revved up. Outside is a small gear which meshes with a larger ("spur") gear that connects to the rest of the drivetrain.
This is simple -- you put fuel here, and as it runs out, you put in more. When the engine is running, a small amount of exhaust goes through a silicone "pressure line" and into the top of the fuel tank to keep the fuel under pressure and flowing continually to the engine.