Downshifting to slow down uses zero fuel. Modern cars cut fuel completely when coasting is detected based on throttle position and whether gear is engaged among other sensors. This is only used a lot in manuals since the torque converter causes freewheeling at lower rpms. It's known as deceleration fuel cut and it's standard in OEM and aftermarket engine controllers.
Typically the engine "restarts" when rpm drops to a bit above idle speed but as mentioned it doesn't happen as much in automatics because of the freewheeling. In automatics it only occurs when deceleration starts at high rpm or the clutch within the torque converter is locked during steady highway driving. I assume shifting into "low" may enable deceleration fuel cut in more situations assuming the driver may be using it to slow down, but I have no evidence of that usage
Typically the engine "restarts" when rpm drops to a bit above idle speed but as mentioned it doesn't happen as much in automatics because of the freewheeling. In automatics it only occurs when deceleration starts at high rpm or the clutch within the torque converter is locked during steady highway driving. I assume shifting into "low" may enable deceleration fuel cut in more situations assuming the driver may be using it to slow down, but I have no evidence of that usage