How it seems to me is that there's a turbo speed sensor and a bit of computer code (probably within the main ECU) to control a valve to bypass air to the rocket, a combustor can. If the turbo speed drops, it's going to mean it's because the throttle's more closed, so it opens the bypass more and adds more fuel, probably via tweaking the engine injectors to make the engine run richer. Carbon monoxide autoignites at 1150ish degrees F, so pretty cold in terms of EGT. If turbo speed goes up too high, it closes the valve moreso and reduces the richness of the engine, eventually transitioning to being how turbos normally work.
At least that's my theory.