Ok, Lenny, here is a (somewhat) simple explanation. The Automatic Gain
Control attempts to keep the volume level the same on all stations so
that if you turn the volume up on a weak station and then tune to a
stronger one, you won't get "blasted" out of the room. The reverse also
holds true - if you've set the volume based on s atrong station and
then tune to a weak one, the volume will be about the same and you'll
be able to hear it.

"Fast" AGC and "Slow" AGC are just what they say they are. One setting
makes it work quickly and one makes it work slowly.

Of course your question is "why" and what does it mean to me?

When tuning the radio, you want a fast AGC to keep all the volume
levels approximately the same so that, if you tune to a weak signal,
you can hear it, and if you tune to a strong signal, the volume level
will not be too great. AGC has been featured on radios since the 1930's
(maybe even earlier) but, on most consumer AM/FM radios, it is so
common that no one mentions it any more (just like the "number" of
transistors - no one "cares" any more). But on shortwave, not only is
it very useful, it is necessary to be able to adjust it. You cannot
take advantage of a radio's DXing abilities unless you have an
adjustable AGC (among other things!).

Once you have tuned in a station and have decide to listen to it, you
switch to slow AGC. Why? Because signal strength fluctuates greatly on
SW signals and a slow AGC, not responding as fast as fast AGC, will
tend to keep the volume level the same and will not fluctuate up and
down as it would if you used a fast AGC. (You'll hear background "hiss"
rising and falling as the signal-strength increases and decreases -
look at your S-meter, but the volume level of the broadcast will stay
about the same.)

So, to make a long story short, when tuning your radio, use fast AGC.
Once you find something to which you want to listen, switch to slow
AGC.

Now what about AGC "off?" You use that in conjunction with your RF gain
control when chasing the most exotic and weak signals (it is also
sometimes useful when listening to Hams on SSB). The AGC can attenuate
(lessen) the apparent strength of some signals so, by turning it off
and adjusting the RF gain to maximum, you may be able to hear the
station. Note that you do NOT want to casually tune with the AGC off.
You'll get "blasted!" (Just try that and see!) You CAN tune slowly in,
say the tropical band, where you might expect to find some unusual or
difficult-to-hear country. But always you must do this carefully. You
can also (rarely) use this to minimize "overload" from a strong station
located close to your location; in this case, you would turn the RF
control all the way down. With an ICOM IC-R75, though, you should
rarely, if ever, need to do this. Remember, most AGC sections,
certainly the one on your ICOM, work extremely well and rarely, if
ever, cause problems that would make you want to turn the AGC off.

So, in conclusion, you will use the AGC fast setting for tuning and the
AGC slow setting for listening. AGC off will be used rarely, if at all.

I hope that's of some help to you.

Best,

Joe