...what happens?

I rigged up a air/fuel mixture meter (just a voltmeter coming off the O2
sensor) a few months back.  On our recent trip across the country, we
obviously spent some time driving in hillier country than we usually do, and
I noticed something funny.  While engine braking down a long hill, all the
clear air coming through the exhaust system, while obviously hot, is
apparently not quite as hot as the sensor needs to be, so that it will
eventually cool down to the point where it no longer provides good output
and behaves like it does right after startup.  After driving for a little,
it warms up again and output is normal.

I'm wondering what the ECU does during this stage.  Does it know to start
ignoring the oxygen sensor if it thinks its output doesn't make sense?  Or
does it just keep chugging along, thinking everything is peachy, and
consequently send too much or too little fuel as the oxygen sensor fails to
react dramatically enough to the changes in ratios of the exhaust gasses?

-Wes
1.9l westy