For combustion you need an ignition source, fuel and oxygen, and your engine does a damn fine job of making sure there is enough fuel and oxygen, and the ignition comes from the spark plug. To pop/bang there needs to be combustion after the engine (ie in the exhaust system),and for that you need fuel, oxygen, and heat is the ignition source. A standard car won't pop and bang as although there is plenty of heat in the exhaust system, there is little oxygen and fuel in the exhaust gasses as they've been burned up in the engine. With some tweaks to the ecu it can ensure there is more than normal amounts of petrol in the exhaust gasses when you let off, but you still lack oxygen so you might get a little pop when the exhaust gasses meet the outside air. What SAS does (Secondary Air System) is inject air into the exhaust system...so with the ecu tweak giving fuel, the exhaust having natural heat, and the SAS providing the oxygen you have an environment where combustion can occur, so you get pops and bangs which is the petrol igniting in the exhaust system. If any of the three components is missing, you get no pops and bangs. We know your ecu can do the petrol, and we know your exhaust gasses are hot enough, so if your car doesn't pop and bang it is most likely your SAS system not working.
