When the engine is first turned on, it will not immediately output 10 RF/t. Its output will scale up to 10 RF/t as its stored energy increases, reaching maximum at 3,749 RF stored (100 C heat).
If the Redstone signal is shut down, the engine will stop outputting power, and its stored energy will be lost at a rate of 10 RF/t. The engine will not stop consuming its current item of fuel, but it will not start burning another item of fuel unless the signal is restored.
If the engine is not connected to an appropriate power consumer and accumulates more than 3,750 RF (about 107 C heat), it will shift to a lower-power mode and reduce its output to 3 RF/t. It will however not reduce its fuel consumption. The engine will not shift out of this lower-power mode until either its fuel runs out, or the engine is manually switched off long enough for its stored energy to drop below 3,750.