When an engine is first started, it needs a little richer fuel-air mixture (i.e., more fuel, less air). In a carbureted engine, this is accomplished by the choke (which restricts the amount of air). In a fuel injected engine, the after-start enrichment device increases the amount of fuel. As the engine warms up, the device gradually reduces the amount of enrichment. Some devices just reduce the amount gradually over time without sensing the temperature of the engine.