They're typically used after combustion, yes. But that's not the only place they work. All the converter is doing is "burning" unused fuel... reacting oxygen with hydrocarbons and carbon monoxide. And really, if you think about it, what are hydrocarbons? They're just unburned fuel. The catalytic converter actually decreases the total fuel efficiency of our vehicle by making even more heat while burning the last of the fuel we didn't use in the engine. It's always seemed like such a huge waste to me. One way to save that heat, though. Put a turbo on the end of the cat. The exhaust can be thousands of degrees leaving the converter... why not use it? I actually think that's how some of the remote turbos are able to function so well. In the old days, pre converters, it seemed well proven that the turbo didn't work well that far away from an engine.