The Venturi effect pulls particulate matter out of the air box and in to the exhaust. What heats up the block is what's called open loop operation. The PCM sees that the ECT sensor (engine coolant tempature) is not at a predetermined temperature. So it increases injector pulse width (inject more fuel) to help accelerate engine warm up. Once the PCM sees the ECT sensor is reading operating tempature, it goes in to closed loop operation; taking in to account MAP (intake manifold absolute pressure), IAT (intake air tempature), TPS (throttle position sensor), and ECT reading to modulate injector pulse width for optimal injection quantity and Timming. That is the most basic description of how the engine warm up works. This applies for all modern automobiles.