A common practice among hypermilers is to shut off the engine when coasting downhill.
Some of the newer cars have this functionality built into the engine management computer. For my 2008 Honda Accord, HOnda says:
Fuel Cutoff Control
During deceleration with the throttle valve closed, current to the injectors is cut off to improve fuel economy at engine speeds over 850 rpm (automatic transmission) or 1,000 rpm (manual transmission).
I guess are cars are sometimes smarter than we think they are.
