When you're engine braking—like when you downshift and let off the gas—the ECU often cuts off fuel to the cylinders. The throttle valve is also closed. In this scenario, your RPMs are maintained by the car's forward motion, which is connected through the drivetrain back to the engine.

So yeah, you're not using any fuel in that case, but you're still turning the engine over. The wheels are essentially driving the engine instead of the other way around. That's how you can have RPMs but no fuel flow during engine braking. The energy to keep the engine turning is coming from the car's inertia.

A common example would be going downhill. You downshift to a lower gear, take your foot off the gas, and let the engine do the work to help slow you down. You'll see the tachometer showing RPMs, but fuel flow is minimal or even cut off, thanks to our friend the ECU.

Look it depends on the age of the car, but let's take an old manual car for example.

On those cars, there's a fuel map to rpm. There's actually a few maps including throttle and ignition timing. But think of a spreadsheet of rpm and fuel at a certain throttle load.

At 0 throttle: The map says to stop the engine from stealing at under say 800 rpm it needs to have fuel added at rpms lower than that to speed up the engine to avoid stalling. At 800rpm it needs a consistent amount kind of a known amount that keeps it in equilibrium. At over 800rpm it needs less fuel the more rpm it has over the idle 800rpm until it's zero fuel.

And you'll feel that, you'll feel that moment the car starts adding fuel because if you're only engine braking to a stop your car will get near that idle rpm and your engine will start adding power to avoid a stall, and your braking will diminish.