Hmmm, I may have an answer for my own question.
The hall sender could detect RPM below or approaching idle speed and add
fuel.

-----Original Message-----
From: pdooley [mailto:psdooley@verizon.net]
Sent: Monday, August 17, 2009 4:45 PM
To: 'Rocket J Squirrel'; 'vanagon@GERRY.VANAGON.COM'
Subject: RE: Not quite a backfire . . .

How does this work?
How does the ECU know to shut fuel completely off when descending but yet
could add fuel in a split second to prevent stalling if say, you press the
clutch pedal in?
The idle switch cannot discern the difference between idling and coasting in
gear.
I really don't see how the AFM could perform this function either.
Somebody enlighten me.

<With no pressure on the gas pedal, the fuel is totally shut off, so the
engine is not firing on any cylinders.....