DFCO is Deceleration Fuel Cut Off. Most modern fuel injected vehicles (including the Honda Jazz) shut the fuel off entirely when you remove your foot from the accelerator. Provided the speed/gear selected is sufficient to keep the engine revs above tick over (actually around 1800 - 2000 rpm) then you use no fuel whatsoever. This is where the ScanGauge comes in handy (Mk 1 Jazz doesn't have instantaneous mpg on the dashboard display). On steeper descents it is better to remain in gear and use no fuel than coast in neutral, and use tick over fuel. The secret of good fuel management is to balance the use of the two, and this is where an instantaneous mpg gauge earns its keep.
Regards steering lock. Try it for yourself. While parked, turn the ignition off but do not remove the key. You can then turn the steering from lock to lock (not easy with no power steering or vehicle movement) and you will see the steering lock doesn't function. However, for EOC you just need to turn the key back one notch, to the Accessory position. That turns the engine off but won't allow you to remove the key.