Originally Posted by Rowan

It works better in undulating terrain. Careful use of the accelerator going uphill, and then putting the vehicle in neutral going downhill.

It is NEVER a good idea to turn off the engine, and I think, for most modern cars, it is impossible to turn off the engine with the ignition key while it is moving above certain speed (see relatively recent articles about stuck accelerators). It works better in manual (stick-shift/standard) vehicles because you have ultimate control over the gears and can use the clutch for instant re-engagement should the need to get out of trouble arise.

It's not a new technique. When I was motoring writer for a newspaper, I did a test drive of a new car that had a challenge of achieving the lowest fuel consumption among the participants -- an economy run, if you like. The difference between highest and lowest was something like 5mpg. This was for cars powered by 1.3-litre engines, so the difference was quite significant.

In fact, economy runs were part of the annual motoring calendar for many enthusiasts "back in the day" here in Australia. Mobil was promoter of one of the events. This was back in the 1960s and maybe early 1970s.