Having the vehicle turn the engine without power is often known as over
running.

The engine basically an air pump and even with the throttle closed some air
will always get pulled through. When things are working properly, the
Vanagon ECU will cut off the injectors when the throttle is closed and the
engine is over run above ~1,350 rpm for the 1.9  and 1,450 for the 2.1. With
no fuel and a lot of air, the engine will cool down. When the engine temps
goes below ~120f the ECU will go back to open loop and the fuel cut off is
no longer active.
Also, when the engine returns to normal idle or the throttle is opened the
O2 sensor is ignored for a short while. Folks with the 1.9 know this effect
when they get that idle surge. The 2.1 addresses this with the raised cut
off speed.

One problem of operating the engine for a long period this way is that after
some time oil will get sucked into the intake valves through the guides.

Sometimes during this scenario you will get some unburned fuel and oil into
the exhaust and you may get some small backfires. This is an unusual
operating condition.

Dennis